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PRECIOUS STONES.
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B. Cryptocrystalline Varieties.
All
these consist of a mixture of silica in a colloid or opaline form, and
silica in some crystalline form, which has been termed Quartzine, and
without or with added impurities.
1.
Chalcedony may be regarded as the mixture defined above, without any
impurities. It is, so to speak, the foundation material from which, by
the intermixture of various other substances, all the further
cryptocrystalline varieties of silica, as Agate, Heliotrope, Carnelian,
Jasper, etc., result.
The
origin of the name is difficult to trace. It seems to have come to be
applied to the mineral we now know under this name by a slowly evolved
confusion, for the Chalce-donius of Pliny would appear to be Dioptase
from the copper mines of Chalcedon. The Leucachates of Pliny and Iaspis
of Theophrastus would seem to be, in part at any rate, our Chalcedony.
The
colour is characteristically a white or greyish-white, but minute
traces of impurities are sufficient to alter the colour, when
distinctive names are often applied. The lustre is waxy to slightly
greasy. Not being of a definite crystalline structure like Quartz, it
lacks the special properties of Quartz dependent on that structure ;
further its specific gravity is rather lower, 2-59—2-60.
The hardness only equals about 6-1/2. Its fracture is flat, and often
rather splintery. It is transparent in only a few cases, more often
sub-transparent, or only translucent. It has no definite crystalline
form of its own, though often found pseudo-morphous after other
minerals. In intimate
p.s.
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