In general, crystalline solids expand on being heated, and the amount of expansion is different in different directions. The expansion in volume is equal to the sum of the linear expansion along the principal axes, thus showing that the crystal form of a stone will have some bearing on the heat effects. But this is a matter for those who can experiment in laboratories. Here wc will note the effect of heat on gem stones in general, for in recent years, attempts to alter the color of certain varieties by heating have been on the increase. Some success has been obtained, mostly at the expense of experiment. The theoretical causes of many of such changes are not yet fully understood.
In some instances, it may be the result of a re-arrangement of molecular structure; in others, some chemical change may take place. For instance, if the color is due to a minute portion of included organic matter, heat would permanently destroy the original color. If the color is due to inorganic matter, such as a metallic oxide, the original color may return on cooling, or it may be changed altogether. But the general effect of heat is to discolor stones.
If a very high temperature is reached, most gem stones are completely spoiled. Those which develop pyro-electricity have already been noted. Others, which change color when certain temperatures are reached, are discussed under their individual headings. For instance, sherry colored topaz loses its color completely on being heated, yet on cooling it changes to an attractive pink. Many of the pink topaz on the market have been artificially “pinked” by packing in magnesia, charcoal, or Plaster of Paris, and then being slowly heated. Incidentally, this is a practice seldom carried out now with topaz, and most of the heat treated stones are found in old mounted jewelry. The stones are often foiled to give extra depth of color.
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