Minerals and Metals in Ancient India v2.c43.s3.t3.1

v2.c43.s3.t3.1

We would now discuss the family of gem minerals containing beryllium, variously coloured, specially green and blue, and having similar hardness. In terms of chemical composition, a sharp distinction has to be made between chrysoberyl (hardness 8-1/2) BeAl₂O₄, a mixed oxide, and beryl (hardness 7-3/4) Be₃Al₂Si₆O₁₈ a cyclo-silicate. The difference in their hardness could be easily tested using another gem, topaz having an intermediate hardness of eight. In absolutely pure form, both chrysoberyl and beryl are colourless. Their colours in the gem form arise on account of lattice substitutions by other elements as specified in Table 7.2. Due to chromium and iron, chrysoberyl samples assume green and yellow colour respectively, the more usual green colour often merging into golden yellow.