Minerals and Metals in Ancient India v2.c43.s3.t8.2

v2.c43.s3.t8.2

If the colour of chalcedony is uniformly red, it is called carnelian; sard is yellowish-red or brown. Yellowish colour is due to hydrated ferric oxide, and the red due to dehydrated ferric oxide. Iron-containing light yellow chalcedony, agate or sard, when exposed to sun or heat, loses water from the hydrated ferric oxide, and gets converted into red carnelian. This technique must have been practised in the Indian quarries of agate and carnelian over several millennia.