The composition of rutile is TiO2. It contains 60% titanium and often also contains iron, niobium and tantalum. Rutile, brookite, and anatase are the three mineral forms of titanium dioxide. The crystals are prismatic and striated, and they sometimes appear as fine acicular crystals inside quartz. Twinning is common, and it sometimes appears as blocky aggregates. It can be reddish brown, red, yellow or black with light brown to light yellow streaks. It has transparent to opaque diaphaneity, a sub-metallic to adamantine lustre, a specific gravity of 4.23, a Mohs scale hardness of 6.0–6.5 and conchoidal to uneven fractures. It exists as a secondary mineral in many magmatic rocks, schist and gneiss. It sometimes appears as acicular inclusions in quartz, corundum and other transparent minerals.