Introduction

Portugal is an important leading player in natural stone production and trading. The national natural stone sector has been experiencing growth over the last 50 years, namely the sub-sector of ornamental stones, whose exports have increased over the years (DGEG 2020). In 2019, ornamental stone made up half of the total value of the exported national mineral resources (DGEG 2020), which shows the relevant status of the sector to the national economy. Also, despite the recent critical COVID-19 situation, Portugal is quoted as an excellent exception of the 2019 critical scenario, having more than doubled its sales over the previous decade and increasing its exports by 25.3% over 2018 (Montani 2020). As stated by Montani (2020), Portugal’s leadership has been confirmed by the exports of simple finished products, a good use of second-choice materials that satisfies buyers sensitive to the price factor.

These positive results contrast with the reduction of operating quarries and workforce in recent years, but are a reflection of the technological advances that have been implemented in each plant. In recent years, many efforts have been made by academics and practitioners to integrate the natural stone sector into the Fourth Industrial Era, also known as Industry 4.0 (da Silva and Gil 2020), and to invest in innovation, internationalisation, R&D, design, and qualification (Silva et al. 2016). Given the traditional and fragmented character of many companies in this sector (Frazão 2018), the need for a transition to a digital economy has become clear for its subsistence and competitiveness. Similarly to other industrial sectors, the main takeaway of having more available data on each step of processing is the possibility of constant analysis, resulting in an overall higher efficiency with less waste. This, along with qualification of the workforce and improvement of work safety conditions, results in a clear economic valorisation (VALORPEDRA 2013; Carvalho et al. 2013a).

Another key factor for success is the market trend regarding stone aesthetics. When talking about ornamental stone, aesthetics is a major factor for commercial selection and a critical market requirement (Luodes et al. 2000; Carvalho et al. 2008; Carvalho 2011; NSI 2016). The perception of stone appearance results from the combination of its features, such as colour, texture, structure, discontinuities, and weathering degree. Colour is a major factor in marketing demand, and the market value varies with trends overtime. In such case, for one-coloured rock deposits, any colour variations due to veining, cracking, or inclusions decrease the market value of a stone. For multicoloured natural stone deposits, an appropriate colour variation and an overall homogeneous appearance are required (Luodes et al. 2000). However, for other buyers, stone’s singularities may be highly valued and hence the market value. Apart from appearance, other factors such as availability and exploitability, workability, and durability are relevant and used as selection criteria to consider when exploring a building stone (Shadmon 1993; Luodes et al. 2000; Carvalho et al. 2008; Carvalho 2011; Manjunatha et al. 2014).

Despite the subjective character of stone selection based on aesthetics and its dependence on the market and visual trends overtime, a more homogeneous and uniform stone is usually preferred (Luodes et al. 2000). Thus, the natural stone sector aims at producing and selecting defect-free stones, since those “defects” induce and constitute heterogeneities and discontinuities in the rock. They will not only penalise ornamental stone in terms of aesthetics but also influence the exploitability of rock masses (Carvalho et al. 2008, 2013a; Carvalho 2011) and, in most cases, the physical and mechanical properties of the natural stones, jeopardising their durability, quality, and performance.

During processing, defect detection of stone products is a time-consuming task, highly dependent on the operator expertise, and mostly done without a geological basis. Some recent advances in image analysis and stone processing have been achieved over recent years, pointing out the benefits of automatic recognition of natural stone surfaces (Sousa and Caldas Pinto 2004; Bonifazi et al. 2006; Kachanubal and Udomhunsakul 2008; Alper Selver et al. 2011; Akkoyun and Fuat Toprak 2012; Martínez et al. 2013; Bianconi et al. 2015; Ferreira and Giraldi 2017; Iglesias et al. 2018; Kardan Moghaddam et al. 2018; Haileslassie et al. 2019; Tereso et al. 2020; Sidiropoulos et al. 2021). The majority of such works focuses on colour and texture (Sousa and Caldas Pinto 2004; Alper Selver et al. 2011; Akkoyun and Fuat Toprak 2012; Kardan Moghaddam et al. 2018; Haileslassie et al. 2019; Sidiropoulos et al. 2021). Some of them link those features to a dataset with the traditional names of ornamental stones (Kachanubal and Udomhunsakul 2008; Bianconi et al. 2015; Ferreira and Giraldi 2017; Tereso et al. 2020), while others try to assess the stone quality (Bonifazi et al. 2006; Martínez et al. 2013; Iglesias et al. 2018). For natural defects in ornamental stone, Proença et al. (2016) applied a methodology based on wavelets for automatic detection of defects in limestone slabs, defining their location and delimitation. Regarding stone non-uniformities, Yarahmadi et al. (2017) established a quality factor considering three factors: shape quality (α), aesthetic quality (β) and discontinuity coefficient (γ). It can be applied to stone blocks or products from stone processing plants, namely slabs and tiles (2D and 3D).

Further image analysis, aiming not only for the automatic detection of stone type but also for its “defects” is gaining more notoriety, mainly by applying machine learning algorithms. As stated, it is very important to start by using criteria and classifiers that are based on a solid geological foundation. Thus, this paper aims to: (i) systematise and discuss the meaning of natural defects and singularities of stone materials; and (ii) systematise and sort relevant classifiers of natural stone heterogeneities and discontinuities, analysing their applicability and discussing potential constraints. The proposed classification system is intended to overcome the subjectivity of defect detection, and can be used by the operators as well as in technological operations for pattern and singularities detection.

Concepts of natural stone heterogeneities and discontinuities

Definitions

Natural heterogeneities and discontinuities are commonly found in the explored rock mass and are taken into account when selecting stone blocks. During processing, after sawing blocks into stone slabs and preparing them for further cutting, operators manually identify defective areas and select the pieces to be obtained from that slab to avoid waste. However, “natural stone defect” has not a consensual definition and depends on one’s personal or industrial background (Proença et al. 2016). According to the Glossary of Boletim de Minas (2019–2020), a defect or a flaw is defined as the absence of one or more features in a stone product which makes it non-conform with the required specifications. Venkat Reddy (2002) defines natural defects as those that influence the quality of a rock deposit. Defects or flaws, according to Ashmole and Motloung (2008), are those features (cracks, joints, veins, banding, or accumulations of a single mineral or colour) present in stone blocks in a small proportion that affect merely their appearance, rendering them unsuitable for current market requirements. Proença et al. (2016) define a defective area on a stone slab as any area with a high degree of local contrast (darker, clearer, or both) and variations in colour saturation. Defects, regarding shape, are local manifestations that appear as linear or as an area of any shape, with varying degrees of filling (Proença et al. 2016). Yarahmadi et al. (2017) categorise stone defects into two groups: one that is more troublesome and contains “unfilled/low-strength-material filled discontinuities” (Palmström 2001) and another that only jeopardises the appearance of a stone product and includes high-strength filled discontinuities and all the other non-uniformities established by Venkat Reddy (2002).

Rocks are natural materials with discontinuous, inhomogeneous, anisotropic, and non-elastic character, also abbreviated as DIANE (Hudson and Harrison 1997). Inhomogeneity, or heterogeneity, is a property that induces variability in the rock mass, resulting in different properties at different locations. An anisotropic material, on the other hand, has distinct properties in different directions. Rock mass’s discontinuities are planes of weakness such as joints, bedding planes, or fissures/fractures that weaken the strength of the rock. These terms relate because a rock’s anisotropy is caused by its heterogeneity and discontinuousness (Hudson and Harrison 1997). Natural defects in a natural stone are then a group of heterogeneities and discontinuities, represented by points or planes of weakness that compromise the stone’s quality, durability, and aesthetic.

The terms defect and singularity are difficult to distinguish and, therefore, they should be used with caution. Singularities are inherent heterogeneities and/or discontinuities (veining, fissures, pits, texture, colour, and other persistent discontinuities) in a stone that may (or may not) compromise its quality and durability, but which makes the stone material unique. Thus, singularities can add value to a stone or, conversely, can make it unsuitable for market requirements based on fashion trends (Ashmole and Motloung 2008). A variation in colouring or texturing, for example, is a common stone occurrence that, while not a defect (Emser Tile 2013), can be regarded as such in terms of market requirements.

Previous works and categorizations

Stone heterogeneities and discontinuities are referred, most of the times, in the (limited) literature available as “stone defects”. Table 1 provides an overview of those typologies and their characteristics, scales and type of products involved.

Table 1 Heterogeneities and discontinuities (mentioned as “defects”) according to the literature
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The majority of authors consider colour variation as a “defect” since is a relevant criterion for trading and depends greatly on the target market (Manjunatha et al. 2014). In addition, textural characteristics, such as crystallinity degree, grain size and geometrical relationship between minerals, also influence and reflect the mineralogy and the rock genetic process. Textural variations are a peril aspect and are normally avoided for commercial purposes (Manjunatha et al. 2014). Discontinuities and structure features are also frequently mentioned since they influence the exploration of a rock mass and, as a result, the resulting block size (Carvalho 2011; Manjunatha et al. 2014).

Venkat Reddy (2002) proposes a vast set of heterogeneities and discontinuities based on geological aspects and creates, as far as the authors are aware, the most comprehensive research work for exposing those features affecting ornamental stone. However, because the studied rock deposits are primarily granitic, other heterogeneities and discontinuities are lacking. Alliance Minerals (2014) and Proença et al. (2016) present a simple classification of “defects” that lacks geological detail. Cosi (2015) provides a visual representation of the most common heterogeneities and anisotropies for various lithologies, which is particularly useful as a visual guide. Yarahmadi et al. (2017) considers two groups of stone “defects”. The first group (“unfilled/low-strength-material filled discontinuities”) is problematic because these “defects” reduce the quality of the deposit, its exploitability, the size of stone blocks, and the processing efficiency (Carvalho et al. 2008; Yarahmadi et al. 2015). In turn, the second group includes “defects’’ that can increase a rock’s aesthetic quality, but they mostly degrade it and are highly dependent on trends and fashion. The classification proposed by Yarahmadi et al. (2017) is effective, comprises a geological base-knowledge, and balances a stone’s competence with its aesthetic quality. However, distinction between the two groups of discontinuities based on filling material or lack of it might induce errors. Despite low-strength-material filled discontinuities lead to lower quality (Plamström 1995; Turichsev and Hadjigeorgiou 2017), the mechanical behaviour of a rock varies greatly with other factors apart from the nature of the filling material (e.g., ISRM 1978; Shang et al. 2016), such as their thickness, hardness, cohesion, and swelling nature (Palmström 1995). As an example, a tight unfilled aperture of discontinuities (problematic according to Yarahmadi et al. 2017) might not greatly compromise the shear strength of a rock (ISRM 1978; Bell 2007). Second, veins are known to control fracturing of intact rock (Turichsev and Hadjigeorgiou 2015; Pereira et al. 2021), determining the strength of the overall veined rock. In fact, veins with softer infilling material lead to lower strength (Turichsev and Hadjigeorgiou 2017), but other factors such as geometry, thickness, terminations, presence of rock bridges, and variations of mineralogy within the vein will also play a role in the mechanical behaviour of a rock (Turichsev and Hadjigeorgiou 2015; Shang et al. 2016). Despite the influence of veining in rock mechanics, it is known that veins present higher tensile strength than joints (Turichsev and Hadjigeorgiou 2015), which is in line with Yarahmadi et al. (2017) scale for the discontinuity factor.

Yarahmadi et al. (2017) present a quality factor formula for evaluating the uniformity within stone products such as slabs and tiles. The quality factor is the first quantitative index available to assess the quality of a stone product. It takes into account a shape factor, whose value is equal to one for slabs and tiles, the aesthetics of stone pieces, and any existing discontinuities that subdivide the stone product. Small heterogeneities and discontinuities in a stone product with uniform distribution are not considered defects and are not accountable for the quality factor calculation. Regardless of the usefulness and applicability of the quality factor during stone processing as a quantitative measure, the authors’ aesthetic quality scale is somewhat subjective and highly dependent on the stone expert and operator.

Classification system for natural stone heterogeneities and discontinuities

Development of the classification system

In this paper it is presented a classification where the terms “heterogeneity” and “discontinuity” will be used instead of “defect”. Moreover, this systematisation takes into account the macroscopic and microscopic descriptions (EN 12407:2019), information found in geological dictionaries (e.g. Allaby 2020) and in other international standards (e.g. ISRM 1978; ASTM C119-04a 2004).

The proposed classification system is based on three main classifiers—colour heterogeneities, textural and structural heterogeneities, and discontinuities—in order to provide flexibility in their application. These classifiers are, in turn, divided into more specific sub-classifiers, being some of them further detailed in sub-groups (Fig. 1). Sub-classifiers and further sub-divisions should be used whenever possible. Moreover, given the variability of stone materials and the usual coexistence of heterogeneities and discontinuities, a combination of sub-classifiers is advised. Figure 2 shows a roadmap for implementing the proposed classification system.

Fig. 1
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Proposed Classification System – main classifiers and further sub-divisions

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Fig. 2
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Natural stone heterogeneities and discontinuities classification system – steps prior to its application and recommendations for its use

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Colour heterogeneities classifier

Alteration

is described as a “defect” by Venkat Reddy (2002) since it affects the strength and durability of the rock (e.g. Anon 1995; Fookes et al. 1988; Basu et al. 2009; Sousa 2013). However, in the current classification system, it is considered that a colour heterogeneity is caused by the formation of secondary minerals (e.g. Pires et al. 2014; Dias et al. 2020; Freire-Lista et al. 2022a, b; Vazquez et al. 2022). As this type of alteration should be described (EN 12407:2019), we recommend linking the nature of mineral inclusions (e.g. oxides, clay minerals, chlorite) with the weathering degree, whose classification is proposed by Anon (1995). As highly weathered natural stones were previously discarded on previous stages, the weathered degree for stone plates, which should be based on the classification for uniform materials (Anon 1995), will most likely only vary between I and III degrees (Anon 1995).

The colour heterogeneities classifier is divided in mineral concentrations and inclusions, each subdivided as described below.

Mineral concentrations

correspond to irregular concentrations, as lenticulae or clots (rounded), of a rock’s composing minerals. They include mineral patches or blotches, that usually stand out from the surrounding rock. Patches can be due to:

  • Darker minerals concentrations (e.g. mafic minerals – hornblende, biotite, pyroxene) and lighter minerals (e.g. felsic minerals – quartz and feldspar; carbonates) (Casal Moura 2000; Casal Moura et al. 2007). This category includes accessory minerals, mentioned by Venkat Reddy (2002) as a “natural defect”.

  • Megacrysts: megacrysts also induce a variation in texture, but for systematisation purposes, are included in the colour heterogeneity classifier. The term megacrysts can be applied to describe the texture of a rock where coarser crystals stand out from the matrix, leading to colour inhomogeneity. They can be further designated as phenocrysts for igneous rocks and as porphyroblasts in metamorphic rocks.

  • Schlieren: usually found in igneous rocks and described by Venkat Reddy (2002) as a “natural defect”. They correspond to irregular flow layers, which mineralogy is the same of the host rock, but they occur in different proportions leading to colour variations (EN 12670:2019; Allaby 2020).

  • Re-crystallized, sparry zones and dismicrite (according to Folk classification (1962), it is a limestone where birds eye structures or other sparry calcite structures contrast the micrite-dominated rock).

  • Fossils: former organisms preserved in the rock. Considered as “defective areas” by Proença et al. (2016) and Akkoyun and Fuat Toprak (2012) since they might reduce the quality of a rock.

  • Nodules: minerals from that rock that protrude from the cleavage planes, typical of slates (Ingham 2013).

Inclusions

associated to late minerals that occur as irregular patches, in veins or disseminated. Commonly associated with the “veins” sub-classifier. They are the result of the rock alteration, weathering, or formation process:

  • Mineral inclusions: iron sulphides like pyrite, pyrrhotite, and marcasite are usually found disseminated, in veinlets, or with massive textures. They are prone to oxidation and can be deleterious, especially when pyrite coexists with carbonates (Ingham 2013). Iron oxides are present in most lithologies and are more pronounced along laminations, cracks, cleavage planes, and grain boundaries. Clay minerals and zeolites may also have a deleterious effect on the durability of a rock.

  • Xenoliths/Enclaves: rocks with different appearance and composition from the igneous host rock (Didier 1989; Britannica 2020; Allaby 2020).

Textural and structural heterogeneities classifier

Textural and structural heterogeneities induce heterogeneity in a natural stone. Five sub-classifiers were considered, as described below:

Grain size variability

abrupt variation in grain size, with grains getting coarser or finer. The size classification varies for different lithologies. The general description of IAEG (Anon 1979) for all lithologies might be applied if needed.

Pattern/Structure variability

when the fabric varies abruptly, for example when grains acquire a preferred orientation.

Lamination/laminae

bedding with less than one centimetre in thickness, typical of sedimentary rocks (Bissell et al. 2021). It can have a parallel or angular (cross-lamination) disposition.

Foliation

a pervasive planar and parallel structure that can be primary when results from rock formation processes or secondary/metamorphic. Slaty cleavage (typical of fine-grained metamorphic rocks, namely slates and with a spacing of 0.01 mm or less), schistosity (typical of coarser grained metamorphic rocks, namely schists), and gneissosity (coarse-textured banding, typical of gneisses, normally millimetric to centimetric) are examples of the latter.

According to Palmström (2001), foliation and laminae can be led to parting (separation of a rock into layers and parallel to the mineralogical structure). They have a length of 0.01 m to 1 m (Palmstöm 1995; 2001) and are considered discontinuities by the author. Foliation and laminae are regarded as singularities of a stone in this study. However, if they lead to parting, the sub-classifier “cracks” must be added (see ahead). Both cracks and partings haver a similar length dimension (Palmström 2001).

Banding

a general term for any tabular and parallel layers that differ in colour, fabric, or mineral composition (EN 12670:2019). Typical of some natural stones, including alabaster, some marbles (e.g. onyx marble), gneisses, and other rocks. According to Alliance Minerals (2014), dimensions for lines and bands should be used with caution since bands are 30 cm long and 2 cm wide. However, banding can still be applied to widths less than 2 cm, sometimes in the order of millimetres.

Discontinuities classifier

Discontinuities are considered as those which induce discontinuousness and anisotropy in a natural stone. They can be regarded as defects since they affect exploitability, the block size, and the efficiency of stone processing operations. Five sub-classifiers were considered:

Pores and cavities

they can be regarded as a defect in some lithologies, whereas in others they are inherent singularities of the natural stone (e.g. travertine). The porosity and cavities greatly influence the physical and mechanical properties of a natural stone. Thus, some vuggy or porous stones may not be appropriate for specific uses, such as those where there is a great exposition to free water and humidity. Pores and cavities must be described whenever possible (EN 12407:2019).

Cracks

are also referred to as fractures, which is a general term for all types of discontinuities (ISRM 1978; Palmström. 2001). Cracks are defined as a small, partial, or incomplete discontinuity (ISRM 1978) in a rock that is observed by the naked eye (ASTM C119-04a 2004; EN 12670:2019; DGEG 2019–2020). Cracks are considered defects since they affect not only the aesthetics of a rock (Sousa 2013) but also its mechanical and physical behaviour (e.g. Pereira et al. 2021). Palmström (1995, 2001) distinguishes cracks from fissures based on length – cracks are smaller (from 0.01 m to 1 m) and fissures are small joints (0.5 m to 10 m), mainly without filling or coating. When referring to natural stone plates, slabs, and tiles, the term cracks is more suitable. On the other hand, for a rock mass scale, discontinuities (fissures, faults, joints) must be described using the ISRM (1978) classification, which considers all discontinuities to have zero tensile strength. However, at the rock mass scale, incipient discontinuities (incipient joints, minerals veins, and bedding) should be distinguished from well-developed fractures because their tensile strength approaches that of a parent rock (Shang et al. 2016, 2018). The strength of such discontinuities depends if they are partially developed, locally weathered with residual rock bridges, or locally “healed” via secondary cementation (Shang et al. 2016, 2018).

Additionally, when cracks are too small to be recognized by naked eye, they are referred to as microcracks (ASTM C119-04a 2004; EN 12670:2019; DGEG 2019–2020) or microfractures (Kranz 1983; Anders et al. 2014). Microcracks are described using a microscope and classified as intergranular, intragranular, and transgranular (Kranz 1983; Lawn 1993; Anders et al. 2014). They are usually flat and open and make the youngest natural cracks in outcrops (Freire-Lista and Fort 2017). Other definitions of microcracks are found in the literature; for example, Simmons and Richter (1976), that define microcracks as having a width-length ratio less than 10–2, typically between 10–3 to 10–5, while Palmström (1995, 2001) defines microcracks as having lengths inferior to 10 mm. Microcracks have a width of less than 0.1 mm, according to a recent review by Anders et al. (2014). Microcracks affect the petrophysical and mechanical properties of the rock, as well as its anisotropy. Their orientation and distribution should be assessed prior to quarrying and processing operations since they influence the stone’s durability (Freire–Lista and Fort 2017). Thus, microcracks should be considered rock defects and classified according to Palmström (1995, 2001).

Veins and veinlets

a planar body formed in a rock by the filling of open spaces and the sealing of former cracks. The filling nature is very variable, usually of quartz, carbonates, iron oxides, and others. Veins produce colour heterogeneity in the rock, but they can be planes of weakness that influence the mechanical and physical behaviour of the rock (Turichsev and Hadjigeorgiou 2015; Shang et al. 2016; Pereira et al. 2021). As a result, they are considered as a discontinuity sub-classifier.

Intrusive structures

recognised in rock masses on an outcrop’s scale. The reason for considering it as a sub-classifier here is that traces of these structures may be found on stone slabs. They comprise mineral intrusions (e.g. quartz, feldspar, calcite, epidote, hematite) or rock intrusions (e.g. pegmatite, magmatic and sedimentary dykes and dyklets).

Stylolites

serrated cracks formed by pressure dissolution under a deformation state commonly found on carbonate rocks. They are typically filled with calcite, clays, pyrite, and/or oxides. They can be characterised based on their geometry (Park and Schot 1968). They are a sub-type of crack/vein but are mentioned separately due to their common occurrence in limestones and marbles, affecting their overall quality (e.g. Aly et al. 2018).

Contact zones

(Venkat Reddy 2002) can also be regarded as a discontinuity but its perception occurs at a rock mass scale, not being a suitable classifier for building stone slabs and tiles.

Qualitative description of discontinuities classifier and sub-classifiers

If users require a more objective description of the discontinuities, a descriptive analysis is provided for cracks and veins.

Table 2 summarises the microscopic description of discontinuities according to EN 12407:2019. Some of these characteristics are difficult to assess on a macroscopic scale, but the high resolution of acquired images in automatic recognition may allow for the use of this detailed description. As stated before, for a rock mass scale, discontinuities (fissures, faults, joints) must be described using the ISRM (1978) classification, considering aside the case of incipient discontinuities (Shang et al. 2016, 2018).

Table 2 Microscopic description of discontinuities according to EN 12407:2019
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Discontinuities can be classified in a variety of ways, therefore, an overview of the subject allowed for the development of a general description, which is depicted in Fig. 3. For open discontinuities, they are classified as fissures, cracks and microcracks according to length (Palmström 2001). The term aperture is applied for open and unfilled discontinuities, as suggested by ISRM (1978), while the term width is used for filled discontinuities. For stone slabs and tiles, the focus of this study, open discontinuities can vary from “very tight” to “wide”. Moreover, for natural stone products, an adaptation of Driscoll's (1995) classification could be used. The author assesses the width of cracks in concrete structures, relating this feature to damage categories ranging from 1 to 5. (Driscoll 1995). Regarding veins and filled discontinuities, Brzovic and Villaescusa (2007) define “weak” veins as those thicker than 2 mm. Furthermore, veins frequently contain more than one type of mineral. According to Brzovic and Villaescusa (2007) and Turichsev and Hadjigeorgiou (2017), “weak”/ soft veins are those with less than 60% of hard minerals, i.e., minerals with a Moh’s hardness value greater than 4 (Brzovic and Villaescusa 2007). Soft veins result in lower rock strength values (Shang et al. 2016; Turichsev and Hadjigeorgiou 2017), but they cannot correspond to the “low-strength-material filled discontinuities” defined by Yarahmadi et al. (2017) since the competence of the infill is only partially related to its hardness. The strength of the infill, which limits rock resistance, greatly depends on the nonuniformity of the mineral structure (cleavage, joints, inclusions) (Kovaleva 1974). Furthermore, some minerals evaluated as hard, such as pyrite, are highly reactive and compromise the durability of a rock (Lindqvist et al. 2007). Thus, not every hard mineral within a vein or infilling a crack guarantee a greater quality. Flah et al. (2020) presented a research work focused on the detection and classification of cracks in concrete structures using image processing and deep learning methods. Five classifiers are considered: no cracks and other four based on cracks’ orientation – horizontal right (HR), horizontal left (HL), vertical right (VR) and vertical left (VL). An adaptation of the former classification is proposed here to the description of natural stone’s discontinuities orientation.

Fig. 3
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Classification of discontinuities according to some description features (adapted from EN 12407:2019)

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Turichsev and Hadjigeorgiou (2017) determined, in studies involving intact veined rock specimens, that vein orientation with respect to the direction of loading had a relevant role in rock strength, while mineralogy and thickness had a minor influence. Veins oriented between 12° and 40°, with respect to the direction of loading, lead to fractures’ propagation along those veins. Moreover, anisotropic rocks present a minimal compressive strength when the anisotropic planes are oriented 45° to the direction of loading (Brown et al. 1977). These aspects are relevant, mostly after ornamental stone is applied and subjected to solicitations.

Despite not being applied in this research, the summary provided by Fig. 3 for the qualitative description of discontinuities aims to provide an objective framework to future studies attempting to assess the damage related to discontinuities (veins and cracks).

Validation of the classification system with portuguese ornamental stones

To establish some constraints of the proposed classification system, seventeen examples of Portuguese natural stones (Casal Moura 2000; Casal Moura et al. 2007; Carvalho et al. 2013a, b) will be revisited. All stone varieties—granites, limestones, and marbles—are referred according to the European nomenclature (EN 12440:2017). Figure 4 shows the location of their main quarries.

Fig. 4
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Adapted from the Geological Survey Geoportal of the National Laboratory of Energy and Geology (https://rop.lneg.pt/rop/)

Location of the main quarries exploring the selected stone varieties – granites (red), limestones (blue), and marbles (orange). 1- Cinzento Ariz; 2—Cinzento Pinhel; 3—Azul Limaga; 4 – Rosa Monção; 5—Rosa Monforte; 6—Amarelo Macieira; 7- Lioz and Encarnadão Chainette; 8- Encarnado da Pedra Furada; 9—Vidraço Moleanos; 10—Moca Creme; 11—Alpinina; 12—Brecha Portuguesa; 13—Ruivina Claro; 14—Branco Anilado; 15—Branco Vigária. Brecha da Arrábida is not explored anymore (no location in the map).

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Granites

Portuguese granites (Fig. 5) can show colour heterogeneities due to mineral patches of the primary mineralogy or because of mineral inclusions. Irregular patches of mafic minerals (Fig. 5b, c, g) are seen in the greyish varieties Cinzento Ariz (Fig. 5a and b) and Cinzento Pinhel (Fig. 5c and d), which are medium grained-size two-mica granites, as well as in the pinkish varieties Rosa Monção (Fig. 5g) and Rosa Monforte (Fig. 5h), rich in K-feldspar. Granites’ red tonalities usually result from iron oxidation of the primary minerals (e.g. Pires et al. 2014; Freire-Lista et al. 2022b). Euhedral megacrysts of feldspar exist on both Cinzento Pinhel (Fig. 5d) and Azul Limaga (Fig. 5e) varieties. Mineral inclusions of oxides along grain boundaries and other discontinuities, as well as transformation of feldspars to clay minerals originate the yellowish granites (e.g. Amarelo Macieira – Fig. 5f), which correspond to weathered and oxidised varieties (Sousa and Lourenço 2015; Freire-Lista et al. 2022a). Moreover, this type of granite is more affected by cracking (Freire-Lista et al. 2022a), which increases its colour variation (Sousa and Gonçalves 2012), and decreases its quality and durability (Sousa 2013).

Fig. 5
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Portuguese granites and their heterogeneities: Cinzento Ariz without heterogeneities (a) and with mafic patches (b); Cinzento Pinhel with local mafic minerals patches (c) and rare feldspar megacrysts (d); Azul Limaga with elongated feldspar megacrysts (e); Rosa Monção granite with an irregular mafic patch (f); and Rosa Monforte variety with overall homogenous appearance (g); Amarelo Macieira showing oxidation (mineral inclusions) (h). Mf – mafics; Fp – feldspar; Ox – oxides (mineral inclusions); mgcrysr – megacrystal; ∆gr – grain size variability

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Limestones

The colour heterogeneities in Portuguese limestones (Fig. 6) result from a dismicrite texture, fossils, or from a chemical variation in the carbonate (e.g. Dias et al. 2020), which might induce a two-toned appearance. The popular Lioz variety (Silva 2019) shows these features (Fig. 6a–d). The two-toned appearance varies from cream to rose (Fig. 6a), cream/rose to yellow (Fig. 6b) or is reddish to yellowish for Encarnado Pedra Furada (Fig. 6d). Rudists are the Lioz main visible feature (Fig. 6a, b, d) for with-the-bed cut, while stylolites are typical for across-the-bud cut, as of Encarnadão Chainette variety (Fig. 6c). The variety Vidraço Moleanos (Fig. 6e and f) shows fossils, sparry calcite zones, and oxidation along/in discontinuities (cracks, veins, stylolites), combined with grain size variability (Fig. 6e) or pattern variation due to different proportions of the calcite components. Moca Creme limestones have a homogeneous appearance (Fig. 6g), or a parallel laminae and cross-lamination (Fig. 6h), depending on the cutting orientation. Stylolites are typical of carbonate rocks and are visible on both Lioz (Fig. 6c) and Alpinina (Carvalho 1998) (Fig. 6i and j) varieties. The latter is strongly tectonised, showing veins with distinct nature, cracks and fissures, and stylolites (with carbonates and/or iron oxides). Cavities and pores are recognised in several limestones, sometimes filled with carbonates (e.g. Fig. 6e and f). The brecciated limestone varieties, such as Brecha Portuguesa/da Ota (Fig. 6k) and Brecha da Arrábida (Fig. 6l) (Kullberg and Prego 2019), present a heterogeneous appearance, which combines colour variation, grain size variation, fossils, and discontinuities.

Fig. 6
figure 6

Portuguese limestones and their heterogeneities and discontinuities: Lioz variety, with fossils and a two-toned appearance (a), with an open crack and yellowish rudist (b) and across-the-bed cut, showing stylolites (Encarnadão Chainette) (c); Encarnado da Pedra Furada variety exhibiting colour variation (d); Vidraço Moleanos limestone, exhibiting different features, such as fossils, sparry zones, and a pattern variation given the limestone elements variety (e), or calcite veins and oxidised zones (f); Moca Creme limestone with an uniform appearance showing different calcite elements (g), and with semi-parallel laminae (h); Alpinina variety showing distinct discontinuities, such as stylolites, cracks and veins, as well as oxidation and recrystallized calcite (i and j); Limestones conglomerates with a heterogeneous appearance, namely Brecha Portuguesa/ da Ota (k) and Brecha da Arrábida (l). Ox – oxides (mineral inclusions); Sparr – sparry zone; Sty – stylolite; Sty – stylolite; ∆clr –colour heterogeneity; ∆gr – grain size variability; ∆ptt – pattern variability

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Marbles

The most prominent feature of Portuguese marbles (Fig. 7) is the veining. Ruivina Clara variety, a veined grey-coloured marble (Fig. 7a), and Branco Anilado, a white veined marble (Fig. 7b) are frequently used. This contrasts with the homogeneous appearance of some other varieties, such as the Branco Vigária (Fig. 7c). Other features, such as inclusions, stylolites, pores, and cavities are frequently found in marbles.

Fig. 7
figure 7

Portuguese marble varieties and their heterogeneities and discontinuities: Ruivina Clara marble, with dark-coloured alternating with white-coloured veins (a); Branco Anilado marble, with colour heterogeneities given the white and greyish veining and patches (b); Branco Vigária marble, with a uniform appearance and a minor oxidation zone (c).Sty – stylolite; Ox – oxides (mineral inclusions)

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Summary

Figure 8 summarises the aspects found for Portuguese granites, limestones, and marbles. With the exception of the “colour heterogeneity” classifier, which has a wide range of applications, all features are described at the sub-group level. Some characteristics are exclusive to the stone type, such as megacrysts and mafics in granites and sparry zones in limestones. Cracks and veins are transverse to all types of stones, notwithstanding their absence in the exhibited granites. The proposed system is comprehensive enough to be used in stones from all over the world, including traits that have not been verified in Portuguese stone varieties. Nonetheless, the examples establish the majority of the features of the proposed system.

Fig. 8
figure 8

Key aspects of the seventeen Portuguese stone varieties. N—number of varieties analysed

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Colour heterogeneities can coexist with textural and structural heterogeneities and discontinuities, implying the overlap of sub-classifiers/sub-groups. Megacrysts, for example, cause colour heterogeneity as well as grain size diversity in granites (e.g. Fig. 5e). The same is true for different carbonate components on limestones (e.g. Fig. 6e). Mineral inclusions (oxides) frequently combine with discontinuities (cracks, stylolites, veins, pores) in all sorts of stones. Veining is primarily a discontinuity, although it can also be interpreted as colour heterogeneity.

Constraints of the proposed classification system

The main constraint of the current classification system is the overlapping of some classifiers, especially for highly variable stone surfaces. While combining sub-groups is advantageous because it provides a more detailed identification of heterogeneities and discontinuities. A single heterogeneity can be classified by more than one distinct sub-classifier in some cases. In such scenarios, the more obvious sub-group should be used or, otherwise, both sub-groups should be mentioned, specifying that they refer to the same heterogeneity. When using this system in artificial intelligence algorithms and when the previous recommendations cannot be applied, then a more general classifier must be chosen.

Selecting a classifier retains some subjectivity in addition to requiring some basic geological knowledge. Thus, operators and future researchers who use this system should consult with geoscientists in advance to ensure consistency and understand the aesthetics of each heterogeneity and discontinuity. In addition, while analysing a natural stone, the examination and use of this system always involve a critical sense: (i) the type of polishing and finishing influence the final aesthetics of a surface; and (ii) if heterogeneities and discontinuities are distributed uniformly and are inherent to the rock, they should not be considered rejection criteria. For multicoloured stone types, the colour heterogeneity must be adequate (Luodes et al. 2000). For example, the Alpinina limestone variety (Fig. 6i and j) is strongly tectonised, which distinguishes it and increases its value, as marbles’ veining is often their singularity (e.g. Fig. 7a). Nonetheless, those and other heterogeneities must always be mentioned to avoid disputes between suppliers and the contracting party, since they may affect the stone’s quality and durability, constraining its usage for specific applications.

The roadmap to follow before applying the proposed classification system and recommendations for its use are summarised in Fig. 2.

Conclusions

The following conclusions have been reached:

  1. 1.

    Instead of "defects," the terms "heterogeneities" and "discontinuities" should be used to minimise any disputes between the manufacturer and the client.

  2. 2.

    The proposed classification system is a preliminary step towards standardising and categorising relevant geological aspects (heterogeneities and discontinuities) to be described for stone product surface analysis. It is a qualitative approach for stone slabs and tiles that can be considered an addition to the quality factor established by Yarahmadi et al. (2017).

  3. 3.

    Uniform discontinuities and heterogeneities in the rock should be regarded as a singularity rather than a defect. Thus, unless this variety does not meet market requirements, the stone product should not be discarded for these reasons.

  4. 4.

    The overlapping of some classifiers for a same heterogeneity implies: (i) select the more prominent sub-classifier/sub-group; or (ii) refer to both sub-classifiers/sub-groups. If not possible, the user should turn to a more general classifier. Agreements on nomenclature between users and geoscientists should be made in advance to ensure consistency.

As future research, the authors propose the use of the present classification system by stone operators as well as in automatic recognition. This would help to more thoroughly validate the classification system and outline that sub-classifiers’ combination is the best approach when sub-classifiers overlap. In addition, discontinuities should be further characterised in order to qualitatively assess the damage related to veins and cracks.