12 
I. THE PRECIOUS 
The distinguishing peculiarities of these minerals are 
hardness (7—10°), density, transparency, lustre, and capa¬ 
bility of polish, combined with beautiful, pure colours ; 
they also generally possess a high specific gravity. 
The elements of which they are composed are usually 
very simple : in the diamond pure carbon (C) ; in the ruby 
and sapphire pure alumina (Ah 0 3 ), coloured by a little 
oxide of iron or manganese ; in the emerald and beryl 
silicates of berylla, or glucina, and alumina ; in spinel man¬ 
ganese and alumina; in the garnet silicate of alumina, oxides 
of iron, and manganese; in the topaz siliceous alumina, with 
aluminate of fluorine, and so on. 
The colour of most precious stones is rather accidental 
than essential, since, with the exception of the garnet, they 
are all quite colourless when pure. They are, however, all 
highly valued and especially adapted for ornaments on 
account of their colour, lustre, and transparency. Others 
are used, on account of their hardness, for engraving and 
perforating soft stones, glass, etc., or as sockets for the pivots 
of watch wheels. Many can he split in certain directions, 
as, for instance the diamond in the four directions of the 
octahedral planes, the topaz and emerald in the direction 
of the straight terminal planes, and the planes of fracture 
in these appear to be distinctly foliated ; in others, such 
as the garnet, splitting can be effected only very rarely, 
and the planes of fracture are irregularly crystalline. 
All those that may be split may also break in the same 
direction on being struck, or if allowed to fall. 
The cutting of gems is effected by iron discs, at first 
by means of emery ; in the case of the diamond, however, 
iiamond-spar or corundite, and most frequently diamond 
dust, is employed for this purpose. The polishing is finally 
completed by using finely-prepared oxide of iron, ashes 
of tin, etc. 
STONES OR GEMS. 
The artificial planes or faeettes are always laid on as 
regularly as possible, in order to give the stone a form 
suitable to the purpose for which it is intended, and to 
produce the finest effect. The larger and purer the 
stone is, the more numerous are its faeettes, and the 
price is also much increased. The value varies, moreover, 
with the size, the purity and brightness of the colours, in 
some cases in a simple, in others in a double and fourfold 
ratio. The setting of all fine stones is applied a jour , 
that is, without a base of metallic plate ; less valuable stones 
are provided wfith such plates, and foil is usually employed 
for the purpose. Precious stones do not change on expo¬ 
sure to the air and light, but when set in rings and the 
like, they become gradually worn by friction, according to 
the degree of their hardness, so that the sharp angles and 
edges become somewhat rounded or dimmed. This is 
least observable in the case of the diamond ; it is some¬ 
what more marked in the ruby and sapphire, still more in 
the emerald and topaz, and in the chrysolite and garnet. 
They are not affected by water and the acids, and, with the 
exception of the garnet, are infusible before the blowpipe. 
A few of them change their colour at a white heat; the 
topaz, for example, becomes rose-red, and many garnets 
become black. The diamond at a very high temperature 
is wholly consumed, and changed into carbonic acid. This 
is most easily effected when in a powdered condition. 
The price of the precious stones is regulated by their 
beauty, purity, colour, cutting, and size. It is calculated 
from their size, the standard being a carat, wdiich is equal 
to 3t gr. Troy. The diamond is the most valuable, the 
rough carat of which is reckoned at £4: 10s.; when cut, 
larger stones cost about £9 to £12 per carat. Next to 
the diamond comes the ruby, emerald, sapphire, opal, 
and so on. 
PLATE I. 
Fig. 1-5 Kough, 6-9 Cut Diamonds. 
We place the diamond in the highest rank among the 
precious stones, because it exceeds all others in hardness, 
lustre, and power of refracting rays of light, and it has 
in every age been the most highly valued. The diamond 
crystallises in the regular or monometric system. The 
primary form is an octahedron (Fig. 1), bounded by eight 
equal equilateral triangles. It is harder than the ruby 
(= 10°), and for this reason may be employed for scratch¬ 
ing and perforating it, as well as all other hard stones 
and metals generally. The diamonds or splinters of 
diamonds, which are to be used for perforators or gravers, 
must, however, at least have one natural angle, because 
cut angles are much more easily worn down. 
The specific gravity varies from 3 , 529-3*55, so that it 
is somewhat more than three and a half times heavier than 
water, and nearly of the same weight as topaz and spinel. 
Colour is either entirely absent, or is inclined to a 
yellow, greenish, bluish, rose-red, or brown hue ; there 
are also perfectly black varieties, which have been lately 
found for the first time, in large crystalline irregular grains, 
in Brazil. The most valuable are the white, rose-red, 
and bluish diamonds ; the brown are least valuable. 
In lustre and transparency, when either rough or cut, 
it exceeds all other precious stones, and is only approached 
by white-lead ore and lead-vitriol, and by the octahedral 
oxide of antimony. It refracts light very strongly, twice as 
strongly as glass, and on this account it may be used with 
advantage in the lenses of magnifying apparatus. It further 
has the peculiarity of dispersing the rays of light to a very 
high degree. For this reason, cut diamonds, especially bril¬ 
liants, exhibit all the colours of the rainbow in play, a pro¬ 
perty only possessed by strong flint glass when mixed with 
oxide of lead, but not by the white topaz in similar circum¬ 
stances. This peculiarity is in a smaller degree apparent 
in rose diamonds, and least of all in table diamonds. 
The diamond is not affected by air, water, and other 
solvents, such as acids and salts of potash; it is, how¬ 
ever, combustible in the focus of a large burning-glass and 
