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Some brown zircons lose their color when heated, and become a brilliant white on cooling, at the same time increasing greatly in luster and brilliance. Other pale blue zircons may have their color intensified, which may or may not remain. Rubies and some other stones will change their color, which returns on cooling. Minute faults in some rubies and emeralds have been eliminated by very careful heating, but this is a risky procedure as the stones may crack. Some green tourmalines are rendered more brilliant by heating, while some aquamarines change to a darker blue shade. The same may be said of some golden beryls, which may be changed to a pale blue.

The majority of the carnelians we see on the market have been the subject of applied heat. Most were originally sards, the dark brown having been driven off, giving place to the attractive flesh color. Again, many of the brown and yellow transparent <|iiartz, the consistently misnamed “topaz,” are the product of 11 eating smoky quartz, while the yellow citrine and treacle brown “topaz quartz” are, in many instances, derived by heating ame-lliysts. At about 500° C, many, but not all, South American amethysts will change to a deep, brownish-red or orange color, sometimes approximating to a rich garnet shade. These stones are sold as Madeira Topaz, Spanish Topaz, or just “Topaz.”

A moderate heat is sufficient to make many amethysts and all (Urquoises fade in color. Further, if a stone contains water in its composition, it will be quickly affected. Opal is an example. This stone quickly cracks, loses its opalescence, and is entirely spoiled. Diamond will stand great heat, losing nothing but its superficial brilliancy, which can be regained by polishing. But at a very high temperature, it burns in air and oxygen, producing carbonic anhydride.

Some stones become clouded on heating, some fuse, while others are infusible. The degree of fusibility is important when making a qualitative analysis, and the results of submitting stones to the so-called “dry test,” which involves the use of a blow-pipe, are most necessary to the chemist who is seeking to ascertain the constituents of a given mineral. Some notes on the fusibility of gem stones under these conditions are given in a later chapter.