Lustre.
This
term is applied to the reflection of light from the surfaces of the
specimens ; it is influenced by the nature of those surfaces, some
reflecting more light than others.
There are six forms of lustre which may be applied to precious stones, as follows:—
Adamantine ... The well-known lustre of the diamond.
Resinous ... The lustre of the garnet.
Vitreous ... That of the emerald.
Waxy ... ... Resembling turquoise.
Pearly ... ... Shown in moonstone.
Silky ... ... Like crocidolite.
Each
of the above varieties of lustre may differ in intensity, but the
different fornix of these are more marked in minerals other than gems.
The several other properties of light, as refraction, dispersion, polarisation, and pleochroism
act in different ways with different stones, but it will be unnecessary
to notice them with the exception of the last.
Pleochroism.
Means
the condition of displaying many colours ; a stone is dichroic when it
exhibits two colours ; trichroic when it shows three, which is the
highest grade of pleochroism. This property, well marked in precious
stones, is a useful one, and easy of determination ; a little
instrument, called a dichrois-cope, is used in this investigation. It
consists of a short tube, in which is placed a prism of Iceland spar ;
at one end, farthest from the eye, a little square hole is placed, at
the other a small lens. With this little instrument the dichroism of a
large majority of gems may be discovered, provided they are coloured.
Holding the stone before the little square hole, and looking through
the tube towards the light, two squares will be seen, and if the stone
is dichroic these squares will be of different colours. A ruby will
show two reds of different- hues, while the garnet will give the same
hue to both squares, and the difference between the ruby and the spinel
is at once seen in the same manner. Besides the ruby ; the Bapphire,
tourmaline, emerald, topaz, aquamarine, beryl, chrysoberyl, iolite, and
amethyst, all show squares of different colours, and are thus easily
determined from their imitations.
The remaining physical properties of minerals are, in the study of gems, of little moment, and will not be considered here.
Individual gems will now be described in detail, commencing, of course, with the most important of them all.
DIAMOND.
Crystalline system=cubic.
Hardness=10.
Specific gravity=352.
Lustre=Adamantine.
Cleavage= Parallel to faces of the octahedron, highly perfect.
Composition= Carbon, being chemically the same as graphite (black lead)
charcoal.
Although so different in appearance, in hardness, and in specific
gravity, from the other forms under which carbon is known, yet, when analysed, they
are found to be identical. The diamond may be considered as
perystallized form of carbon, while charcoal and lamp black are
amorphous.
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