History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c17.s3.t1.1

v1.c17.s3.t1.1

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 4.