6-1/2-7-1/2 3.400-4.500
Jet 3-1/2 1.348
Onyx (a variety of Chalcedony) 6-1/2 2.598-2.610
Quartz 7 2.670
Tourmaline (not unlike Black Resin
in appearance) 7-1/4 3.024-3.300

CHAPTER XIII.

VARIOUS PRECIOUS STONES.

_The Diamond._

To recapitulate certain of the facts respecting the diamond.--This
wonderful gem has the distinction amongst precious stones of being
unique; though many are composed of two, three, or but a small number of
elements, the diamond is the only stone known consisting of one element,
and absolutely nothing else--pure crystallised carbon. Its hardness is
proverbial; not only is it untouched by the action of a hard file, but
it occasionally refuses to split when struck with finely tempered steel,
which it often causes to break. Such was the case with the South African
diamond, for when the knife that was to break it was struck smartly with
a steel bar, the first blow broke the blade without affecting the
diamond, yet a piece of bort, or diamond dust, splinters, or defective
diamonds (all these being called bort), may readily be pulverised in a
hard steel mortar with a hard steel pestle.

The diamond is the hardest stone known; it is also the only stone known
which is really combustible. It is of true adamantine lustre, classed by
experts as midway between the truly metallic and the purely resinous. In
refractive power and dispersion of the coloured rays of light, called
its fire, it stands pre-eminent. It possesses a considerable variety of
colour; that regarded as the most perfect and rare is the blue-white
colour. Most commonly, however, the colours are clear, with steely-blue
casts, pale and neutral-colour yellow, whilst amongst the most expensive
and rare are those of green, pale pink, red, and any other variety with
strong and decided colour. Although these stones are sold by the carat,
there can be no hard and fast rule laid down as to the value of a carat,
for this depends on the size, quality, and the purity of the stone. The
larger the stone the greater the value per carat, and prices have been
known to range from 25_l._ per carat for a small stone to 500_l._ per
carat for a large one, whereas the exceptionally large stones possess a
value almost beyond estimation.

It often happens that some st