open
workings, but when the Persian mines were under the government of that
country systematic working underground was carried out, and there are
extensive underground workings in New Mexico as well as huge heaps of debris on the surface.
When cut as a gem, it is either en cabochon or
in a flat elliptical plate. The colour is sometimes artificially
intensified by Berlin-blue. Turquois shows its colour well at night,
while most of its imitations appear less beautiful in artificial light.
The best specimens . are known as Oriental Turquois or "stones of the
old rock." The value now is not so great as it was at one time, but
good specimens find a ready market, and on account of the difficulty in
obtaining large perfect pieces the value rises rapidly with the
increase of weight.
746. Gypsum. .
Gypsum
is only used in its finely fibrous variety, Satin Spar, and massive
variety Alabaster. Alabaster does not seem to have been included in
Pliny's Alabastrites, though the fibrous and the stalactitic varieties
of carbonate of lime were. King identifies the Lygdinus with compact
Gypsum or Alabaster.
Gypsum
occurs colourless, white, pink, blue, brown and yellow. Some forms show
banding of different colours. It is transparent to subtranslucent; the
lustre is vitreous to pearly; in Satin Spar it is silky. On heating,
water is given off and the mineral becomes opaque. The specific gravity
is 2-28 to 2-33, the hardness 2. The mineral
crystallises in monosymmetric forms, and there is one distinct cleavage
and two indistinct ones. In chemical composition it is
