Minerals and Metals in Ancient India v2.c47.172

v2.c47.172

*Topaz* : It has the formula [Al (F,OH)₂] AlSiO₄ with variable proportions of F and OH. In an extreme case, it could be (AlF)₂SiO₄. Prismatic crystals. Hardness 8. Vitreous luster. Colour is usually straw yellow, wine yellow. Impurities may cause diverse colours such as grey, green, blue, red etc. Often contains inclusions of CO₂. On heating, the mineral may assume a pink or red hue, resembling the Balas ruby. On heating with cobalt solution it becomes blue.