The peperino rocks: historical and volcanological overview

The name peperino derives from the Italian word pepe (from the Latin word piper, pepper) and has been used in the common language for lithified volcanic deposits characterized by light grey through dark grey tones and granular textures, resembling that of ground pepper. Among these, the best-known examples are represented by some phreatomagmatic deposits of the Colli Albani Volcanic District, near Rome (Italy), and ignimbrite deposits of the Cimini Mountains near Viterbo (Northern Latium, Italy), which have been widely employed in artefacts of historical and archaeological interest. In particular, these resistant volcanic rocks have been widely employed by the Etruscans and Romans since the seventh century BCE to produce sarcophagi and dimension stones, as well as architectural and ornamental elements. These rocks are still in use for building ornaments, street furniture and artworks in central Italy today. In this review, we provide an overview of the use of this term, and an exhaustive review of the different rocks of central Italy defined as peperino, describing their distinctive textural features, as well as their eruptive sources and outcrop areas. Indeed, despite the common macroscopic aspect, peperino rocks can be associated with several different eruptive styles and emplacement mechanisms. Our review is also addressed to archaeologists concerned with restoration initiatives and provenance studies, as well as to volcanologists studying the genetic processes of pyroclastic rocks and related naming conventions.


Introduction
The term peperino (from the Latin root piper, pepper) was originally established in Italy to define peculiar light porous volcanic rocks with a granular texture (resembling ground pepper), used as construction stones since pre-Roman times. Generally speaking, peperino is a kind of "diamictite" (sensu Flint et al. 1960), that is a lithified, poorly sorted, deposit consisting of "floating" clasts suspended in a fine-grained matrix (Menzies 2009). According to the Penguin Dictionary of Geology (1972), peperino is defined as: "A rock of mixed pyroclastic and sedimentary origin, including pyroclastic material, and weathered and eroded volcanic material (including scoriae, cinders, etc.) cemented together". In the Oxford Dictionary (1982) peperino is defined as: "n. light porous (usu. Brown) volcanic rock formed of small grains of sand, cinders, etc.". The French dictionary Le Petit Robert suggests the date of 1694 for the arrival of the word péperin in the French language to refer to a volcanic tuff employed as construction stone in the Roman region. However, Scrope (1827) extended the use of the term peperino to describe clastic rocks from Limagne, in the Auvergne region of central France, which comprise mixtures of lacustrine limestone and basalt and resemble ground pepper. Scrope (1858) interpreted them as having originated by a "violent and intimate union of volcanic fragmentary matter with limestone while yet in a soft state". To refer to the rocks from this type locality, the term peperino then shifted to peperite, which is now commonly used for clastic rocks comprising both igneous and sedimentary components, which were generated by essentially in-place disintegration and active mixing of intrusive magma, lava flows or hot volcaniclastic deposits with unconsolidated, typically wet sediments (Skilling et al. 2002;Sigurdsson et al. 2015).
The term peperino (sometimes also reported as piperino) has been used as a rock descriptor in the international geologic literature and volcanology textbooks (e.g. Rittmann 1967;MacDonald 1972;Kilburn and McGuire 2001). Other major textbooks usually describe peperites, yet seldom mention peperino. For example, Cas and Wright (1988) mention peperino in their chapter 4 on volcaniclastic deposits, while Fisher and Schmincke (1984) refer only to the piperno (another rock term derived from peperino, see below) from the Phlegraean Fields type locality, near Naples, as an example of welded fallout tuff (agglutinate). However, since its early definition, the term peperino has been applied to a variety of volcanic products that, beyond a generic common aspect, derived from quite different genetic processes. Given the widespread use of the term peperino, yet broad sense of its definition, we here complete a full review of the origin and development of the term. We also describe the lithofacies sub-types, as well as the source conditions, compositions and eruption types to which the rock can relate.

Establishment of the term peperino in Italy
In the Italian geologic literature, the term peperino has been applied in referring to several different volcanic rocks of the Latium region of central Italy (Fig. 1) that share similar appearance, as well as similar textural and mechanical features (see Farr et al. 2015). For the most part, they are represented by either phreatomagmatic deposits of the Colli Albani Volcanic District, near Rome, or ignimbrite deposits of the Cimini Mountains near Viterbo (Northern Latium) (Fig. 1). The name peperino derives from the Italian word pepe (pepper), and has been used in the common language since the seventh century BCE, for rocks used to produce sarcophagi and as building stones (Jackson et al.  Jackson and Marra 2006;Farr et al. 2015;Diffendale et al. 2018). They are still used as architectural and ornamental elements today. Indeed, these rocks are characterized by a speckled light grey to dark grey tone and granular texture, resembling that of ground pepper. The same Latin root, piper, is at the base of the local name Piperno (Breislak 1786) for a genetically distinct volcanic rock from the Phlegraean Fields, near Naples, which is a welded lithofacies of the Campanian Ignimbrite (Fedele et al. 2008).
The ancient Romans used the word lapis (stone) for the peperino rocks, as well as for all the hard rocks, either volcanic or sedimentary, employed as dimension stones in building construction. In particular, the two types of rock cropping out in the surroundings of Rome that are today referred to as peperino, i.e. the Peperino di Marino (or Peperino albano) and the Peperino di Gabii, were called Lapis albanus (Vitruvius, De Architectura, 2.7) and Lapis gabinus (Strabo,Geography,5.3.10;Tacitus,Annales,15.43), respectively, by the ancient Romans.
Here, we present an overview of the origin of the term peperino and its use, in both the common and scientific language, as based on an extensive bibliographic search, given as Supplementary Material #1. The original language versions of the definitions, translated herein into English, are also given in Supplementary Material #1. Moreover, we provide a complete review of the rocks of Latium that have been named peperino in the scientific literature, describing their textural aspects, their eruptive centres, areal distributions and type localities. While the family tree for the development and application of the term peperino is given in Fig. 2, examples of peperino rocks are given in Figs. 3 and 4. This sets the guidelines for unambiguous distinction of peperino types and related source areas.

Early definition of peperino rocks
The first attested use of the word peperino in the scientific literature seems to be in an essay by the Swedish mineralogist Johan Gottschalk Wallerius, as reported by Desmarest (1765) (for this and the following citations, see also Online Resource 1 for extended text). Desmarest (1765), however, argued against the similarity with the Tiburtine rock (i.e. the travertine from Tivoli near Rome) proposed by Wallerius, going on to describe pépérine as.
"a compound stone, which is based on a terracotta, which envelops materials altered or not altered by fire; this cooked paste, of a whitish or reddish gray, is more or less friable. There are, mixed in different proportions, scoriaceous and melted materials, a few blades of glass, mica, gabbro or schorl, pieces of limestone, quartz, etc".
Desmarest (1773) then used the same term for the volcanic rocks into which the cellars of the French city of Clermont-Ferrand, in the region of the Chaine des Puys and Limagne graben (Auvergne), were excavated: Today this hill is known to be part of a maar rim on which the cathedral of Clermont-Ferrand stands (Boivin et al. 2015). Several decades later, Scrope (1825Scrope ( , 1827 used peperino for an ensemble of rocks occurring in the Auvergne: "In giving the name of peperino to a volcanic conglomerate consisting of fragments of basalt and scoriae, without pumice, tufa, or any trachytic matter, united either by simple adhesion or a calcareous or argillaceous cement, I follow the Italian geologists, who have continued this trivial term to a similar rock, which also, like that under consideration, occasionally contains fragments of limestone and primitive rocks,bituminized wood,&c.,pp. 45,47". Ten years previously, the French naturalist Cordier (1815) had used the term Pépérite to identify tuffs. Originally, the term pépérite was thus closely related to the term peperino as used by the Italian geologists and archaeologists. For some time, the two terms were used as synonyms by French geologists and applied to special types of volcanic tuffs and breccias found in the Limagne (Guérin 1839).
Since the nineteenth century, the term pépérite has suffered a strong "semantical drift" (De Goër De Herve 2000), being extended to other kinds of rocks produced by a wide range of geological processes, and not exclusively those connected to volcanism. The French pépèrites represent a standalone group of rocks and are not the subject of this review. Instead, a modern volcanological description and classification of the pépérites is provided in White et al. (2000) and Skilling et al. (2002). White et al. (2000) proposed the adoption of peperite as a genetic term: "… applied to a rock formed essentially in situ by disintegration of magma intruding and mingling with consolidated or poorly consolidated, typically wet sediments". Breislak (1786) provided another definition of peperino as being like "a fragmented lava", while Ferber (1776) used piperino (from the Latin root piper) to name the volcanic rock outcropping in the surroundings of the Albano and Nemi lakes (southeast of Rome, Italy). He classified the rock as tufo (tuff) and described it as "greenish-grey rusty ashes, with black sherd-lamelles, white garnet-like sherds, and small pumice-stones". Ferber (1776) also stated that "The quarries of Piperino, employed at Rome in buildings and sculpture, are near Marino", allowing us to unambiguously identify it with the Lapis albanus. Ferber (1776) also mentioned.
"Piperino or Granito di S. Fiora; a particular sort of lava, composed by a large quantity of white sherl showing the macroscopic textural and compositional features of Lapis albanus. 1 = lighter, olive-grey glass matrix (with zeolite cement), 2 = numerous lava rock fragments, 3 = leucite and analcime crystal fragments (hexagonal shapes), 4 = biotite (black mica) crystal fragments, 5 = limestone rock fragments; smooth, fairly durable stone surface in oblong parallelepiped crystals; much black sherlmica, and some lava". However, he specified: "Properly the name of Piperino belongs only to indurated volcanic ashes or tufo with sherl-crystals; accordingly, the above-described of S. Fiora, being a lava, should not be called by that name".
Both Ferber (1776) and Breislak (1786) also discussed the similarity of the rock locally called piperno occurring at Pianura, near Naples (in the Phlegraean Fields Volcanic District, Fig. 1). This rock was extensively used since the Greek-Roman age to pave roads, and was the main building stone in the Campania region until the cessation of quarry activity at Soccavo and Pianura at the beginning of the twentieth century (Calcaterra et al. , 2005. The name piperno derives from the Latin name of the Roman village Pipernus (modern Priverno, southern Latium) which, in turn, is a distortion of the Latin term piper (pepper). The local use of the word piperno can be traced to the earliest written documentation dating to 1428 CE (de' Gennaro et al. 2000;GeoPortale 2009). This rock is a proximal deposit of the Campanian Ignimbrite, the highest magnitude explosive eruption of the Mediterranean area in the last 40 ky (Fedele et al. 2008;Scarpati et al. 2020). It is exposed along the eastern sector of the caldera rim of the Phlegraean Fields and in the city of Naples (Rittmann 1950;Perrotta et al. 2006;Fedele et al. 2008;Scarpati et al. 2020) and consists of alternating beds of welded tuff with flattened scoriae (fiamme) and coarse lithic breccia with grey lava clasts (Fedele et al. 2008). Von Buch (1809) distinguished the term peperino from "tuff" as follows: "It is easy to distinguish the peperino from the tuff. In the first almost all is fresh, perfect and without damage. In the second, all is dull, dead and destroyed [here Von Buch (1809) remarks on the relatively An accurate description of the petrographic and lithologic features of the "Peperino albano" was provided in Gmelin (1814b): "Peperino is a fragile stone, very fit to building, of an earthy texture, and not heavy. It seems to consist of a congeries of very different bodies, particularly fragments of augite of a dirty green, dark green mica, iron sand, compact limestone, basalt, and a kind of lava very resembling pumice. These seem to be agglutinated by an earthy cement. Sometimes it contains fragments of stones. These fragments are always sharp-edged, generally small, but sometimes weighing several pounds, especially those consisting of basalt and limestone. Sometimes, though rarely, fragments of feldspar, and a scoriaceous matter of a dark green color are mixed into the peperino".

Scientific classification of peperino rocks
The first classification of the peperino rock type within the volcanic petrographic nomenclature is provided by Brocchi (1817) in his "Catalogo ragionato di una raccolta di rocce per servire alla geognosia" (Geological catalogue of rocks). Brocchi (1817) classified three types of volcanic rocks: lava, tufa (tuff), and peperino. Moreover, he distinguished several lava varieties, including "lava necrolite", "lava piperno", "lava nenfro", and "lava sperone", all of which have relationships, to different degrees, with the peperino rocks. We note, though, that most of these rocks, with the exception of "lava necrolite", are not effusive products and thus do not comply with the modern definition of lava (cf. Sigurdsson et al. 2015), as discussed below.
Brocchi ( It is worth noting that the remark on the unaltered character of the peperino with respect to tufa mirrors the distinguishing criterion already outlined by von Buch (1809).
Based on the above criteria, Brocchi (1817) described several peperino rock types from different areas of centralwestern Italy in his rock catalogue (Table 1). All had the common characteristic of a highly heterogeneous componentry, comprising juvenile angular fragments of massive and/ or vesicular lava or scoria, and wall rock fragments (roccia primitiva). In addition, free crystals and fragments of pyroxene, haüyne (lazialite), leucite (amfigena), magnetite (ferro magnetico), Ti-magnetite (ferro titanico) and brown to black mica were identified. These were found in the rock along with angular clasts of Apennine limestone, with or without tremolite, and were supported by a calcareous matrix. Due to the abundant calcite veins found in some places, Brocchi (1817) advocated an origin from underwater eruptions for the peperino rocks.
Brocchi (1817) also argued against the improper use of the term peperino when applied to the volcanic rocks cropping out near Viterbo, in the Cimini Mountains, "distinguished by the local inhabitants with the word peperino" (i.e. Peperino delle Alture Auct., Peperino tipico) (Peperino of the heights; Typical Peperino), "as it has been already adopted in books to indicate a particular variety of tuff".
Indeed, Brocchi (1817) identified these rocks from Cimini as a lava, which he named "lava necrolite" (from the Greek words necros, death, and lithos, rock), on account of the use by the Etruscans to carve their sarcophagi and excavate their burial chambers into this rock type. However, Brocchi (1817) also introduced the name "lava piperno" for the welded ignimbrite deposits cropping out in northern Latium at Ronciglione and Caprarola (part of the Vico volcano), based on the close resemblance with the piperno occurring at Pianura, near Naples, described by Breislak (1786). It is not clear whether Brocchi was aware of the fact that the terms piperno and peperino should be considered synonymous. With the definition of "lava piperno" and the statement that it is cognate to the "lava nenfro" (see below), Brocchi (1817) created the scientific basis linking all rocks included in Table 1 to the term "peperino".
Brocchi (1817) also used "lava nenfro" (from an untranslatable local word) to apply to two welded ignimbrite deposits cropping out in northern Latium, i.e. at Villa San Giovanni in Tuscia (near Blera and Vetralla) and in between Table 1 List of Peperino rock types, also including lava rocks reported with the local name of peperino/piperno, from the "Catalogo ragionato di una raccolta di rocce per servire alla geognosia" by Brocchi (1817). Site of occurrence (see Fig. 1  On the small road that leads from Castel Gandolfo to the Fishermen's House, located on the shore of Lake Albano Peperino filled with angular fragments of brown and reddish lava. It is a variety of peperino similar to that of Lake Giuturna Albano Crater 1st eruptive cycle, unit b2 (Freda et al. 2006); lava-like At Lake Giuturna Grey peperino with bits of brown, yellowish and tobacco coloured lava, and with pyroxene, some amphigenae and pebbles of Apennine limestone On the eastern side of the Aricia Valley basin, in the place known as the Petrara Peperino of grey-black colour, very solid, heavy, mixed with large quantities of more or less porous brown lava fragments, which look like grains of lapillus tightly joined together. It also contains fragments of reddish and yellowish lava, large crystals of greenish pyroxene, large laminas of black mica, some amphigene and pieces of primitive white limestone. The large quantity of the above mentioned fragments of lapillus differs from the ordinary peperino Nemi Crater 1st eruptive cycle On the lower, southwestern rim of the Aricia Valley Grey Peperino, composed of a combination of fragments of pyroxene, pieces of grey-yellowish and reddish lava, and fragments of white limestone, with some amphigene and some mica laminas. It is very similar to ordinary peperino.
It differs from this, in addition to the ingredients, in that it is insensitive to the magnet, when the other is very attractive Nemi Crater 2nd eruptive cycle It also contains bright feltspates and pyroxene particles. It is analogous to that of Rota "Peperino listato" (Sabatini 1896); "ignimbrite B" (Locardi 1965) or "ignimbrite II trachitica" (Bertini et al. 1971) from Vico volcano LAVA SPERONE On the top of the Tusculum above the Camaldoli. In that place, it constitutes a large cliff, and is excavated like a chisel stone, which is employed in Frascati and in the neighbourings Yellowish in colour, arid, with a coarse granular texture, scattered with whitish globes, which seen with a lens, are known to be amphigene, and with very minute cubic crystallines of hyacinth colour Welded scoria lapilli of the Villa Senni post-caldera phase of activity from several vents along the Tuscolano-Artemisio caldera rim (Fornaseri et al. 1963;Marra et al. 2009) Lava spero, Pietra sperone, Pietra gabina [improp.] (Lugli 1957) At Villa Falconieri near Frascati Yellow-brown, solid, with a lumpy granular texture, and in some places slightly porous, with vitreous amphigene Canale and Rota (Fig. 1). These rocks were exploited locally for building and sculpture stone, with Brocchi (1817) stating that "this rock is nothing but a lava that holds a middle place between the lava piperno and ordinary lava". However, these are not lavas. The first deposit (later named "peperino listato" by Sabatini 1896) is actually a welded facies of "ignimbrite B" (Locardi 1965) or "ignimbrite II trachitica" (Bertini et al. 1971) from Vico volcano. The second deposit is a valley-confined, lithified facies of the Bracciano pyroclastic flow unit of the Sabatini Volcanic District (de Rita et al. 1993), also named locally as peperino (Bertini et al. 1971).
The term nenfro is also used locally in the Vulsini Volcanic District (southern Tuscany-northern Latium; Fig. 1). Moderni (1903) first used nenfro to name welded ignimbrites characterized by the occurrence of flattened black pumices (fiamme) in a dark grey to reddish-purplish, strongly lithified ash matrix (Alberti et al. 1970;Nappi and Marini 1986). These deposits were emplaced during the Paleovulsini activity at ca. 0.6-0.5 Ma (Vezzoli et al. 1987;Palladino et al. 2010).
Finally, Brocchi (1817) defined "lava sperone" as an intermediate rock type between compact and porous lava. This term was introduced by Gmelin (1814) after the local name for the welded scoria lapilli fall deposits (from Strombolian activity) exploited around the Tuscolo archaeological site along the Tuscolano-Artemisio caldera rim of Colli Albani. The term "sperone" (spur) has been erroneously reported by Lugli (1957) as an archaeological name for the Lapis gabinus, probably because of a faint resemblance between these genetically and petrographically different rocks (Farr et al. 2015).
In Table 1, we report all peperino rock types listed by Brocchi (1817), with the identification of the corresponding stratigraphic units from the modern geological literature. Besides the canonical Lapis gabinus and Lapis albanus, which correspond to the phreatomagmatic deposits of the Castiglione maar and of the Albano multiple maar (second eruptive cycle), respectively, Brocchi (1817) lists a number of other peperino rocks. These include pyroclastic deposits of the Albano (first eruptive cycle) and Nemi multiple maars (Colli Albani), as well as of the Tomacella eruptive centre, located near Frosinone (central-southern Latium) in the Volsci Volcanic Field (Cardello et al. 2020;Marra et al. 2021). These rocks all have a common phreatomagmatic origin and similar field appearance. For example, they are all characterized by carbonate and lava clasts in a lithified grey ash matrix containing loose clinopyroxene, dark mica, leucite and/or feldspar crystals.
In addition, Brocchi (1817) classified as peperino some distal occurrences of the pyroclastic-flow deposits of the Tufo del Palatino to the east of Rome (Table 1). In the same way, the popular definition of peperino was attributed to the same unit in the central and northern sectors of Rome (i.e."Peperino della Via Flaminia","Peperino grigio"; see sheet 149-Roma of the geological map of Italy; Table 2). Brocchi (1817) rejected the use of the term peperino for other volcanic rocks of northern Latium, which instead he considered as different varieties of lava, with the only exception being the deposits between Rota and Tolfa, which he termed peperino. These rocks correspond to a valley-confined, lithified facies of the Bracciano pyroclastic flow unit of the Sabatini Volcanic District (de Rita et al. 1993) and are named locally as peperino (see also Bertini et al. 1971).
Following the acceptance by the scientific community of the classification proposed by Brocchi (1817), the term peperino was subsequently used to describe volcanoclastic rocks outside of Italy. Scrope (1825), for example, proposed the term "calcareous peperino" to label the characteristic alternations of beds of carbonates with those containing abundant volcanic fragments (see above) as occurring at the Gergovie plateau (Auvergne, France; also cf: Chazot and Mergoil-Daniel 2012). Scrope (1825) also used "calcareous and conchiferous Peperinos" for similar sedimentary successions of Veneto (i.e. Veronese, Vicentino, and Euganean hills; northern Italy) and southern Sicily.
Roth (1887)  More than one century later, Le Maitre (2002) reviewed the term "peperin-basalt", describing it as "an obsolete name for a tuff which forms mud flows and contains large crystals of augite and hornblende".
However, apart from some sporadic uses outside of Italy, such as those mentioned above, until the end of the nineteenth century the term peperino remained generally a name used in the volcanic areas of Colli Albani, Sabatini and Cimini-Vico. The diffusion of this term to the Roccamonfina and Vulsini volcanoes was proposed by Moderni (1904), who aimed to.
"demonstrate that the origin of this tuff, so widely represented among the materials of the Tyrrhenian volcanoes, can be as different as its position is different in different locations".

Tufi stratificati varicolori di Sacrofano
From cineritic to lapillaceous, mainly incoherent. Some levels have a notable thickness, if grey lithoids are locally called "peperini" Grottarossa Pyroclastic Sequence (Karner et al. 2001a, b;Farr et al. 2015) Grey brownish to purplish tuff, rich in flattened scoria if close to its point of emission ("fiammae"), rounded in the more distant areas. If cemented, locally called "piperno di Mazzano" Welded facies of Tufo Rosso a Scorie Nere Sabatino (de Rita et al. 1993) or Tufo Grigio Sabatino (Campobasso et al. 1994) Tufo di Bracciano Mainly lithoid known locally as "peperino" "Bracciano pyroclastic flow unit" (de Rita et al. 1993) Predominantly lithoid grey tuff, called (locally) "peperino", rich in fragments of lava, scoriae and sedimentary rocks Phreatomagmatic products of the Baccano crater  Finely stratified grey tuffs made up of ashes with abundant inclusions of volcanites (lava and scoria) and sedimentary rocks (limestone, marly limestone, etc.) belonging to the underlying Flysch formation. Locally called "peperini", when cemented Products of the Hydromagmatic centres  "In the Sabatini Volcanoes this tuff is not part of the materials that ordinarily constitute the cones, but it is found stratified more or less horizontally always at a certain distance from the eruptive centers: in the Roccamonfina Volcano and in the Vulsini Volcanoes, it also contributes to forming the skeleton of the cones". Moderni (1904) Vernia et al. 1995).
In addition, Moderni (1904) described three peperino occurrences near Valentano village, in the area of the Latera caldera (western Vulsini). These are characterized by a grey tone and contain yellowish scoria clasts, lava and limestone lithics. Abundant small leucite, augite and mica crystals are also present. These deposits possibly correspond to a lithified facies of the Tufi di Poggio Pinzo phreatomagmatic-Strombolian succession (Vezzoli et al. 1987;Palladino and Simei 2005).
It was mainly in the twentieth century, the term peperino entered in the official geologic literature as an informal name for a number of pyroclastic deposits (Fig. 2). In Table 2a-d, we list all stratigraphic units for which the local definitions of peperino (or, in a few cases, piperno) have been reported in the 1:100.000 geological mapping completed by the Servizio Geologico d'Italia (Geologic Survey of Italy). The term was also endorsed by two volcanological studies for the Latium region in the 1960s (Fornaseri et al. 1963;Mattias and Ventriglia 1970).
Following Mattias and Ventriglia (1970), the geological map of the Sabatini Volcanic District (de Rita et al. 1993) then also used the piperno di Mazzano label for a welded facies locally found in the lower part of the Tufo Rosso a Scorie Nere. This unit is distinct from the peperini listati, which represent deposits from an earlier eruptive event in the western Sabatini (see also Cioni et al. 1993).

Eruptive and emplacement processes of peperino rocks
The term peperino and its derivation piperno have been used in reference to a variety of volcanic rocks that differ in terms of their formative eruptive and emplacement mechanisms, source areas and ages. With the exception of the Peperino delle Alture, which is a lava rock type related to the effusive activity of the Cimini Mountains dome complex, all the other cases of application are for rocks derived from the products of explosive eruptions with different styles and magnitudes (Table 3). There are three cases of peperino application when assessed by eruption style and magnitude: i) for rock types derived from small-scale (< 1km 3 ) phreatomagmatic eruptions. These rocks mainly represent the proximal facies of base surge deposits and are usually lithified, massive to faintly stratified, and rich in aquifer lithics. They can be related to maar-tuff rings, as for the "Peperino di Gabii" from the Castiglione maar in the Colli Albani (Marra et al. 2003), to small caldera systems, as is the case for the peperino examples from Baccano and Montefiascone, or to carbonate-seated maardiatremes, as in the examples from the Volsci Volcanic Field (Cardello et al. 2020;Marra et al. 2021). In other cases, peperino rocks are related to massive, thickened facies of phreatomagmatic pyroclastic currents locally channelized in topographic lows, as is the case for the "Peperino albano" at the type locality of the Marino quarries (Giordano et al. 2002;Freda et al. 2006); ii) for rocks resulting from the emplacement of widespread (1-10 km 3 ) ash-rich, accretionary lapilli-bearing, "wet" pyroclastic currents. This applies to, for example, the Tufo del Palatino, resulting from a large-scale explosive event with a significant hydromagmatic component during the early activity of Colli Albani (Marra and Rosa 1995;Karner et al. 2001a;Palladino et al. 2001); iii) for rocks consisting of fiamme-bearing, welded pyroclastic deposits from magmatic hot pyroclastic currents. These can be associated with small-scale lava dome collapse, as is the case for the "Peperino Tipico" from the Cimini Mountains (Cimarelli and de Rita 2006) or with moderate-to large-scale (1-100 km 3 ) caldera-forming events, which include the peperino and piperno examples from Vico, Sabatini and the Phlegraean Fields (Table 3).

The use of peperino rocks in Roman and Etruscan monuments
In the City of Rome, systematic quarrying of Tufo del Palatino for dimension stone (i.e. a natural rock that has been selected and finished to specific sizes or shapes) began in the archaic period (8 th -third century BCE; Lanciani 1897; Coarelli 1974;Lugli 1957;Cifani 1994). However, the designation of peperino for the Tufo del Palatino (commonly known as "Cappellaccio") has been used only in a few scientific publications (e.g. "peperino grigio", sheet 149-Roma of the geological map of Italy; Ventriglia 1971). The Tufo del Palatino is typically affected by pronounced and rapid weathering due to its relative softness and pervasive cleavage. For this reason, its use as building material ceased at the beginning of the fourth century BCE, when improved quarrying techniques and access to deposits further from Rome allowed the exploitation of more durable rocks. The two peperino rocks, Lapis albanus and Lapis gabinus, are characterized by higher uniaxial compressive strength than all other tuffs from the Colli Albani and Sabatini volcanic districts. As a result, they are less prone to weathering. This favoured their widespread employment in architectural elements that are subject to high pressures, such as piers, weight-bearing walls, and arches (Jackson and Marra 2006). According to Tacitus (Annales, 15, 43), Emperor Nero promulgated an edict that ordered the use of Lapis albanus and Lapis gabinus in the reconstruction of the basements and ground floors of buildings destroyed by the 64 CE great fire of Rome, the reason being that these rocks were reputed as fire-proof. Such a notion must have been retained from earlier times, since Augustus built the retaining wall of his Forum with these two rocks so as to protect it from the devastating fires that frequently occurred in the nearby Subura quarter (Figs. 5 and 6). These rocks were also used to construct the Tabularium in 78 BCE, which hosted the state archive (Lugli 1957). Lapis gabinus was also systematically used for the pillars and arches of ancient bridges and aqueducts (Lugli 1957). A full chronology of the principal Roman monuments in which the two peperino stones (Lapis albanus and Lapis gabinus) have been employed is summarized in Table 4. Here we complete a short review as to how three types of peperino (Lapis albanus, Lapis gabinus and Peperino di Viterbo) became historically and archaeologically important by being used in the construction of ancient monuments in Rome and Latium. This is also relevant for Lapis albanus Lugli (1957) and Holloway (1994) proposed that Lapis albanus began to be used as building material in the third century BCE. However, recent geochemical analysis from the Sant'Omobono Sacred Area demonstrated that this date should be moved back to the early fifth century BCE (Diffendale et al. 2018;Farr et al. 2015). In fact, Lapis albanus occurs in the facing of the firstphase (fifth century BCE) platform supporting the twin temples of Fortuna and Mater Matuta and in part of the eastern edge of the platform at its southern end. The use of Lapis albanus was confirmed for several monuments within the temple platform, spanning the 4th through third centuries BCE (Diffendale et al. 2018). Moreover, Karner et al. (2001b) suggested an early use (fifth century BCE) of Lapis albanus for the Tullianum. Lapis albanus was also extensively used as the base for inscriptions carved in stone, since the oldest known example for Consul Appius in 264 BCE (Torelli 1968) through the first century BCE. Since the mid-third century through the end of the second century BCE, Lapis albanus was systematically used to produce the sarcophagi of Scipions' sepulchre, and was used in construction of several tombs and villas built along the Via Appia in the second and first centuries BCE (Lugli 1957). It was largely used during the late Republican and the early Imperial age in several buildings and temples (Table 4). Lapis albanus was used during the mid-second century CE in the construction of Adrian's temple and mausoleum, the temple of Antoninus and Faustina. Its employment was, however, abandoned towards the end of the second century, its last use being attested in the year 202 CE at the permanent legionary fortress of Castra Albana, close to the original quarrying site at Alba (Lugli 1957).

Lapis gabinus
In the site of Gabii, located on the rim of the Castiglione maar ( Fig. 1), the local deposit (Peperino di Gabii or Lapis gabinus) was exploited since at least the tenth century BCE to produce blocks employed in the construction of walls and long after the Roman conquest in 493 BCE. In the late Republican period, the town of Gabii was depopulated due to the extensive quarrying activity of the Lapis gabinus.
According to Lugli (1957), the earliest use of Lapis gabinus in Rome was in the arches of the Acqua Marcia aqueduct in 144 BCE. However, Farr et al. (2015) identified the occurrence of Lapis gabinus blocks in a small staircase abutting the fifth century BCE main podium of the twin temples at Sant'Omobono, although it was probably added in a later period. Probably due to the greater distance from Rome to the quarry site, with respect to that of the Lapis albanus, Fig. 6 The perimeter wall that divided the temple of Augustus from the Subura quarter is one of the most imposing peperino works of ancient Rome the Lapis gabinus was employed to a lesser extent in the City (Table 4). The most prominent buildings in which it occurs are the state archives (Tabularium), and the Forums of Caesar and Augustus.

Peperino di Viterbo
The Peperino di Viterbo (also known as "Peperino Tipico") in northern Latium was used as a building material, as early  (Bianchi et al. 1963). Peperino di Viterbo (although often reported as "nenfro" in archaeological texts) was also used in a large number of Etruscan sarcophagi (e.g. from the burial chambers of Tarquinia and Tuscania) between the seventh and first centuries BCE, as well as sculptures mainly representing fantastic animals (Bianchi et al. 1963). Lugli (1957) reported erroneously the occurrence of the Peperino di Viterbo in ancient Rome, in the "portichetto" at Foro Olitorio (actually made up of an intrusive rock).

Conclusions
The term peperino was originally used for volcanic rocks of grey tone and granular texture, with resemblance to ground pepper, derived from phreatomagmatic eruptions of Colli Albani (i.e. Peperino di Marino, Peperino di Gabii). However, the term peperino (with the piperno variety) was extended to other volcanic rocks of central Italy that span a range of magma compositions, eruption styles and magnitudes. Peperino examples are mainly related to the potassic and ultrapotassic magmatism of the Roman and Campanian provinces (Middle to Upper Pleistocene), with mafic (K-foidites) to differentiated (trachites and phonolites) compositions, with a few exceptions related to silicic magmatism of Pliocene-Lower Pleistocene (e.g. Peperino di Viterbo from the Cimini Mountains). With one exception, peperino rocks are related to explosive eruptions ranging from small-scale phreatomagmatic events associated with diatremes, maars and tuff rings to moderate to large pyroclastic currents associated with major caldera-forming events ( Table 3). The different magma compositions, eruptive styles and geological settings with which peperino rocks are associated result in a range of componentry. The unifying features of all peperino rocks are the grey tone, the granular appearance, and the lithified ash matrix. However, the juvenile components may vary from dark, poorly vesicular scoria in phreatomagmatic cases to flattened scoria or pumice (fiamme) in welded ignimbrite cases. Early authors, such as Gmelin (1814) and Brocchi (1817), also pointed to the characteristic occurrence of both black (accessory lava) and white (accidental limestone) lithic clasts in some peperino examples (e.g. Peperino di Marino).
Peperino stones play a major role in the history of construction, sculpture and archaeology, yet the term lacks a rigorous geologic definition. Peperino can only be used informally as a generic, purely descriptive term, which cannot be related to a specific composition or volcanic process. In this regard, attempts to use its offshoot term, peperite, remain ambiguous and lack general consensus among geologists.
The reference peperino examples from central Italy volcanoes may be useful for comparison with similar rock types that are common in volcanic environments worldwide. However, we do not claim for a broad use of the term in place of, or in addition to, the existing well-established terminology for volcanic lithofacies. We suggest that peperino can be used mainly in geo-archaeological and cultural heritage contexts to describe a light, porous, poorly sorted pyroclastic rock consisting of lapilli-sized juvenile and accessory and accidental lithic clasts, supported in a lithified ash matrix typically grey in tone. Detailed textural and compositional characterization may allow identification of a specific peperino type and its source area, relevant for provenance studies and restoration interventions.