CHAPTER XXXVII CASSITERITE, ANATASE, PYRITES, HEMATITE CASSITERITE Though usually opaque, this oxide of tin, corresponding to the formula SnO_{2}, has occasionally, but very rarely, been found in small, transparent, yellow and reddish stones suitable for cutting. The lustre is adamantine. The refraction is uniaxial in character and positive in sign, the ordinary index being 1·997 and extraordinary 2·093. The specific gravity is high, ranging from 6·8 to 7·1. The hardness is on the whole less than that of quartz, being about 6 to 7 on Mohs’s scale. ANATASE This mineral, which is one of the three crystallized forms of titanium oxide, TiO_{2}, occurs often in small, brown, transparent stones which occasionally find their way into the market. The lustre is adamantine. The refraction is uniaxial in character and negative in sign, the extraordinary index being 2·493 and ordinary 2·554. The specific gravity varies from 3·82 to 3·95, and the hardness is about 5½ to 6 on Mohs’s scale. PYRITES, HEMATITE These two metallic minerals were employed in ancient jewellery. The former, sulphide of iron, FeS_{2}, is brass-yellow in colour, and has a specific gravity 5·2, and hardness 6½ on Mohs’s scale. It is found, when fresh, in brilliant cubes. The latter, oxide of iron, Fe_{2}O_{3}, has a black metallic lustre, but, when powdered, is red in colour—a mode of distinguishing it from other minerals of similar appearance. Its specific gravity is 5·3, and hardness 6½ on Mohs’s scale. In modern times it has been cut in spherical form to imitate black pearls, but can easily be recognized by its greater density and hardness. Hematite is used for signet stones, often with an intaglio engraving. CHAPTER XXXVIII OBSIDIAN, MOLDAVITE Two forms of natural glass have been employed for ornamental purposes. Obsidian results from the solidification without crystallization of lava, and corresponds in composition to a granite. The structure is seldom clear and transparent, and usually contains inclusions or streaks. The colour is in the mass jet-black, but smoky in thin fragments, and occasionally greenish. Its property of breaking with a keen cutting edge, in the same way as ordinary glass, rendered it of extreme utility to primitive man, who was ignorant of the artificial substance. The refraction is, of course, single, and the refractive index approximates to 1·50. The specific gravity varies from 2·3 to 2·5. The hardness is 5 on Mohs’s scale, the same as ordinary glass. Obsidian is obtained wherever there has been volcanic activity. Vast mines of great antiquity exist in the State of Hidalgo, Mexico.