CERTAIN kinds of Precious Stones are often burnt or subjected to a high temperature, the heat exercising a very peculiar influence upon many stones, and in some cases modifying or utterly changing their colour.
Thus, the Oriental Carnelian owes its beautiful tint to artificial exposure to heat. The Pink Brazilian Topaz, too, derives its remarkable colour from burning.
One way of burning Precious Stones is to roll them up in a piece of sponge or tinder, and set fire to the enveloping material. Another method is to place them in a crucible, with either unslaked lime or iron-filings, and heat them until they are quite clear. Occasionally where a faulty stone with dark spots is burnt with sand and iron-filings, the spots are removed and the colour equalized ; but the process requires great care.
Rubies are occasionally infected with white spots which can be removed by burning. Many coloured Jar-goons have their tints more or less completely discharged on exposure to a high temperature. Smoky Rock-Crystal also, carefully heated in a crucible with lime, sand, or charcoal, will usually come out perfectly clear. The discharge of colour is evidently due to the decomposition of the organic matter, with which the stone was tinted.