PRECIOUS STONES.	259
	394. Zircon. There has been much confusion about the relation to this species of some of the minerals spoken of by Pliny, but it seems well established that the Lyncurium of Theophrastus was our Zircon in part. It is supposed it was the seventh stone, the Ligure, of the Rationale of the High Priest. Zircon is usually found in some shade of brown, ranging from a pale yellowish-brown to a rich red-brown; only
	rarely is it colourless, green or violet. It varies from transparent to opaque, those varieties used as a gem being transparent or subtransparent. The lustre is distinctly adamantine. The mineral is doubly refracting in a marked degree, and the actual deviation is greater than in any other gem stone except the Diamond, the values of the indices for yellow light being 1-923 and 1;968. The dispersion, however, is feeble, so that the mineral when cut shows a lack of fire; the dichroism is very feeble; there is a marked phosphorescence on heating. Zircon is peculiar in showing s 2