Page:Handbook of Precious Stones.djvu/110

94 for stones of volume sufficient to secure accuracy; all the specimens save one were from Ceylon:—

This tabular statement of specific gravities would be incomplete without reference to the change which many zircons undergo when strongly heated—most coloured zircons thus lose or change their original hue. Some become nearly or quite colourless, others from a brown or reddish hue change to a dull green. Now it is found that zircons naturally of high density are not altered in this character by heating, but, on the other hand, that specimens of low density usually contract considerably after having been raised to a high temperature. Thus a green stone, having the low density of 4·0 acquired a density of 4·31 after it had been strongly heated; while a gold-coloured specimen was raised from 4·375 to ·657. Yet so long ago as 1875 I described a low density dull green zircon which suffered no condensation by being heated but retained its original density of 4·02. The recent researches of Dr. S. Stevanović and of Mr. L. J. Spencer point to the existence of three modifications of zircon-material. One of these has a permanent density of 4; another has the same density, which, however, can be raised by heating; a third modification has the permanent density of 4·7. All the different densities and changes of density which have been observed can be explained on this hypothesis.

Several curious phenomena are presented by certain varieties of zircon. For example, all the bright green stones of low density as well as some others of ill-defined greyish yellow and greenish yellow