Note:  Do not rely on this information. It is very old.

Chromium

Chromium (Cr., atomic weight, 52.1), a metallic element, of which the chief natural compound is chrome iron ore, FeOCr203. It also occurs in chrome ochre, crocoisite and some other rare minerals. The colour of emerald and the green in serpentine are due to small quantities of this element. It is a very hard metal of a grey or greenish colour, with a specific gravity 6.8. It forms three oxides, CrO, Cr203. and Cr03. Two series of chromium salts exist corresponding to the two first, but the highest oxide has an acid nature and yields the chromates. The chromic oxide, Cr2O3, is used in preparation of green glasses, enamels, etc., and as a pigment under the name of emerald green. Combined with water it is also known as Guignet's green. The salts of all yield solutions coloured green or violet, and many show a remarkable change from the one tint to the other by warming. A double salt of chromium and potassium sulphate is known as chrome alum (K2SO4 + Cr2(S04)3 + 240H2). This forms dark purple octahedra, largely used in calico-printing, dyeing, and tanning.