Considered from the scientific point of view, zircon is by far the most interesting and the most remarkable of the gem-stones. The problem presented by its characters and constitution is one that still awaits a satisfactory solution. Certain zircons, which are found as rolled pebbles in Ceylon and never show any trace of crystalline faces, have very nearly single refraction, and the values of the refractive index vary from 1·790 to 1·840, and the specific gravity is about 4·00 to 4·14, and the hardness is slightly greater than that of quartz, being about 7¼. On the other hand, such stones as the red zircons from Expailly have remarkably different properties. They show crystalline faces with tetragonal symmetry, the faces present being four prismatic faces mutually intersecting at right angles and four inclined faces at each end (Fig. 78). They have large double refraction, varying from 0·044 to 0·062, which is readily discerned in a cut stone (cf. p. 41), and the refractive indices are high, the ordinary index varying from 1·923 to 1·931 and the extraordinary from 1·967 to 1·993. Since the ordinary is less than the extraordinary index the sign of the double refraction is positive. The specific gravity likewise is much higher, varying from 4·67 to 4·71. The second type, therefore, sinks in molten silver-thallium nitrate, whereas the first type floats. The second type is also slightly harder, being about 7½ on Mohs’s scale. By heating either of these types the physical characters are not much altered, except that the colour is weakened or entirely driven off and some change takes place in the double refraction. But between these two types may be found zircons upon which the effect of heating is striking. They seem to contract in size so that the specific gravity increases as much as three units in the first place of decimals, and a corresponding increase takes place in the refractive indices, and in the amount of double refraction. The cause of these changes remains a matter of speculation. Evidently a third type of zircon exists which is capable of most intimate association with either of the other types, and which is very susceptible to the effect of heat. It may be noted that stones of the intermediate type are usually characterized by a banded or zonal structure suggesting a want of homogeneity. The theory has been advanced that zircon contains an unknown element which has not yet been separated from zirconium. Zircon of the first type favours green, sky-blue, and golden-yellow colours; honey-yellow, light green, blue, and red colours characterize the second type; and the intermediate stones are mostly yellowish green, cloudy blue, and green. It is another peculiarity of zircon that it sometimes shows in the spectroscope absorption bands (p. 61), which were observed in 1866 by Church. Many zircons do not exhibit the bands at all, and others only display the two prominent bands in the red end of the spectrum.