THOUGH usually opaque, this oxide of tin, corresponding to the formula SnO2, 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 I.997 and extra-ordinary 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.
This mineral, which is one of the three crystallized forms of titanium oxide, TiO2, 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 5 1/2 to 6 on Mohs’s scale.
These two metallic minerals were employed in ancient jewellery. The former, sulphide of iron, FeS2, is brass-yellow in colour, and has a specific gravity 5.2, and hardness 6 1/2 on Mohs’s scale. It is found, when fresh, in brilliant cubes. The latter, oxide of iron, Fe203, has a black metallic lustre, but, when powdered, is red in coloura mode of distinguishing it from other minerals of similar appearance. Its specific gravity is 5.3, and hardness 61 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.