History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c17.s2.t3.2

v1.c17.s2.t3.2

Among the semi-precious stones, carnelian was the most popular, not only because of its attractive red colour and translucency but also on account of the easy availability of agate (from Rajpipla conglomerate, Rangpur etc.) which was cooked to produce different shades of red, yellow and lemon carnelian/sard. These are all crypto-crystalline silica belonging to the chalcedony family of minerals. The colour of red carnelian is due to ferric oxide disseminated in the silica matrix, and the yellow and brown to hydrated ferric oxide.