36 GEMS AND PRECIOUS STONES.
This
garnet is said to derive the name of " almandine " from Pliny, who
applied the term " Alabaudicus " to the " carbuneulus " cut and
polished at Alabanda.
These
stones have of late years gone out of fashion, though at one time a
good stone of the size of half-a-crown would fetch as much as £50.
Some
of these red garnets have been fashioned into cups and boxes. The Mayer
collection at Liverpool, England, includes a cup of this material,
while another is in the Hope collection. A delicate cross is sometimes
seen in these stones, the star having four rays.
The
hardness of the almandine garnet is 6'5-7'S, and its specific gravity
3'7-4'21. Its common crystal form is that of the rhombic dodecahedron.
Its approximate composition is SiO—40, AL O3:=20, FeO=35, sometimes with some MnO, MgO, and CaO.
The
precious garnet is found in Ceylon in alluvial deposits, and also in
gneiss; it is also found in India, Brazil, and many other countries.
The
common garnet is found all over the world. It is a brownish red,
subtranslucent, or opaque variety ; its composition is similar to that
of the almandine garnet.
4. Spessartite.
The
manganese-alumina garnet is named from Spessart, in Bavaria. It is not
a plentiful stone, and is not used to any extent in jewellery. It is of
a deep hyacinth or brownish-red colour, having a hardness of 7-7'5,
with a specific gravity of 3'7-4'4. Its approximate composition is Si02=35, Al, 03=14, FeO=14, MnO=35.
It is distinguished by its strong manganese reactions, with the proper tests.
5. Andradite (The Iron-lime Garnet).
Several
garnets are included under the general name of andradite. They have a
hardness above 7, and a specific gravity of 8'41. Their approximate
composition is SiO»=36, Fe2O3=30, MnO=3, CaO=29, and usually some potassium.
Aplome,
colophonite, pyrenitc, a black garnet named melanite, and a transparent
yellow or greenish stone named topazolite, are the principal iron-lime
garnets; they are little used in jewellery, being perhaps
mineral-ogical curiosities more than commercial articles. At present
they are rare.
6. Ouvarovite.
The
lime-chrome garnet was named af ter Uvarof, President of the Imperial
Academy, St. Petersburg. This gem has a fine emerald-green colour, but
it is unfortunately of rare occurrence. "When sufficiently large it is
a beautiful stone, having a hardness of 7'5-8 with specific gravity 34.
It is found in the Ural mountains, associated with chromic iron
(chromite). As chromite is common in this Colony it would be well to
bear ouvarovite in mind. Its approximate composition is SitL=35, AL 03=5, Cr2 03=22, CaO=30, with some iron and magnesia. It gives chromium reactions with the borax bead.
Some
extremely beautiful but rather soft gems of various hues of green to
brownish green, have been impoited into England, for use as ornamental
stones, since the year 1878. They are found in the gold washings of the
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