GEMS AND PKECIOUS STONES OF GREAT BRITAIN. 
125 
in square prisms. Connected with their forms is their refractive 
power : the diamond, spinel, and garnet, or the cubical stones, 
have only simple refraction ; whilst all the others possess double 
refraction, and these latter present very interesting phenomena, 
when viewed in thin sections by means of polarised light, 
according as they possess one optic axis, as the beryl and zircon ; 
or two optic axes, as topaz. A knowledge of the differences of 
the crystalline form of minerals, and of their relative density or 
specific gravity, is of great practical importance in distinguish- 
ing one from the other, as, for example, the so-called British 
diamonds from the real gem, or the false from the true topaz. 
Some minerals, however, used for ornament, are neither crystal- 
line, transparent, nor very hard, as lapis lazuli, turquoise, and 
malachite. 
Although several of the mineral species generally regarded 
as gems or precious stones are known to occur in the British 
Islands, and many of them, especially in olden times, were cut 
and polished for personal ornamentation, the extension of trade 
and commerce over the globe has tended to lessen the estima- 
tion in which they had previously been held, by the intro- 
duction of a supply of similar stones of infinitely finer quality 
from more distant parts. 
Thus we have garnet, topaz, beryl, or emerald, sapphire (?), 
and the varieties of amorphous and crystallised quartz, as rock- 
crystal, amethyst, cairngorm, agate, onyx, calcedony, jasper, 
opal, &c. 
In Grreat Britain these are generally found imbedded in the 
rock mass, whilst the valuable and most highly prized stones 
that come to us from abroad, are rarely obtained from the 
original matrix, but usually are found as grains or pebbles in 
ancient or modern alluvial deposits, the more perfect and solid 
ones only having resisted the wear and tear to which they have 
been subjected since they were eliminated from the breaking 
up of their parent rocks. Hence the comparative abundance 
of the precious stones in the river valleys which traverse the 
metamorphic strata, in which these minerals were originally 
imbedded, as in India, Ceylon, Australia, and South America. 
The minerals and stones which occur in the British Islands, 
capable of being used for ornamental purposes, may be classified 
under the following arrangement : — 
Silica : 
Crystallized; or Quartz 
Colourless: Eock-crystal (so-called Bristol, 
Welsh, Cornish; and Irish Diamonds). 
Yellow 'I 
I Brown I Cairngorm; or so-called False Topaz. 
I Smoky 1 
^Violet=Amethyst. 
