gem. For it presents a considerable range of rich as well as of delicate hues; its surface lustre is brilliant, almost adamantine; while its chemical composition is rendered noteworthy through the predominant presence of the rare earth zirconia. Although zircon, even in its most richly coloured varieties, is but feebly dichroic, yet some specimens display a considerable amount of "fire," owing to their high dispersive power, while the majority depend for their brilliancy mainly upon their strong refraction of light, which approaches that of diamond. Moreover the spectroscope reveals the presence, in many of the transparent specimens, of a series of black absorption bands (discovered by the author in 1866), which are characteristic, and have been attributed to small quantities of a uranium compound, to erbium and to an unknown element.[1] Zircon crystallises in the tetragonal system, and usually occurs in short square prisms terminated by pyramids containing facets of tetragonal octohedra. Its hardness is about 7·5, but rather lower than this figure in the varieties of lower density. The specific gravity of zircon demands special consideration, as this physical property presents, in the case of this mineral, features of quite singular interest. In fact the specific gravity of different specimens ranges from 3·98 to 4·86, a range more extensive than that of any other precious stone, indeed of any other mineral. Of the gem-varieties of zircon the least dense are those which are of a pure leaf-green colour, 3·98 to 4·18; then come the paler and duller green stones, 4·2 to 4·4; then the clouded orange or gold-coloured stones, 4·32 to 4·45; then the great bulk of yellow, orange, red, brown, puce, and white stones, 4·6 to 4·75; the flame-red zircons of Expailly, Auvergne, with a density approaching 4·86 conclude the series. The following set of determinations of the specific gravity of flawless cut specimens of zircon (both jargoons and hyacinths) will be useful for reference, particularly as the numbers represent the values obtained
- ↑ Fig. 11. Frontispiece.
