History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c36.s7.8

c36.s7.8

3. *Lustre, Transparency, Colour and other Optical Properties* :—Difference in lustre is often regarded as a diagnostic feature of the gems. According to Varaha gems of superior qualities should possess cool (lit. waxy) lustre, be limpid and emit rich rays. Classification according to colour is often enjoined. For instance, a diamond may be perfectly colourless, or it may be yellow, black, red or copper-tinted. Stones which are translucent, opaque and of dull colour are inferior in quality. In the case of diamond special stress is laid on its displaying the colours of the rainbow (due to its high dispersive power). Some gems have only one shade of colour, other again two (dichroism).