Minerals and Metals in Ancient India v2.c43.s1.t1.2

v2.c43.s1.t1.2

Table 7.2 provides the list of principal gems which were known and used in the ancient world. These gem minerals have been arranged in the decreasing order of hardness (Moh’s Scale). Energetic chemical bonds, often of the covalent type, result in hardness, which in turn provides the necessary durability of gem minerals. Such materials may have high refractive index and lustre but are usually colourless when completely devoid of impurities. Table 7.2 shows how trace quantities of idiochromatic transition elements such as Fe, Cr, Mn, Ni, Co get into the hard but colourless networks and alter their colour drastically. Nassau (1983: 23 and 333-36) has explained that not less than 13 out of the possible 15 causes attribute colours to minerals and gemstones.