tinguish
between them ; the inferior harduess of the garnet is also useful in
deciding the difference; the specific gravity of both are nearly alike,
so that this test cannot be used with any certainty. The difference
between spinel and coloured topaz ia easily determined by the dichroism
of the latter, and by topaz becoming electric when heated, a property
not possessed by the spinel; the difference of crystalline form is a!so
useful, and even in waterworn stones there is often a cleavage plane on
the topaz sufficient to assist in its determination.
The
higher specific gravity of zircon is sufficient to distinguish it from
spinel, besides it is less hard, has a different crystalline form, and
becomes pale or colourless when heated in the blowpipe flame, while the
spinel becomes darker under this test.
The black spinel (pleonaste) might be mistaken for magnetite, but is not magnetic.
The
spinel is interesting as having been made artificially very
successfully, by heating together alumina, magnesia, and boracic acid
at a very high temperature; the latter has the power of dissolving the
two other constituents, and volatalising when greatly heated; these
spinels were obtained in perfect crystals, and having the correct
hardness. Spinel crystals have also been obtained by other methods.
Spinels
vary much in price. Small stones range from 5s. to 10s. per carat;
medium stones, of fair colour, 20s. to 40s.; large stones, 60s. to
100s. per carat, but they must be of good colour and free from flaws.
The balas ruby varies much in price. Good spinels are valuable stones
and worth searching for.
In
this Colony spinels are sometimes found in alluvial deposits with other
gem-stones; they are usually small, and might be easily mistaken for
garnets, but on account of the superior value of spinels over the
latter stones, it is advisable to be certain as to the identity of red
stones, if of any size or of good colour. When spinels arc found in a
sedimentary deposit, they were originally derived from older rocks. The
following are the principal localities from which the spinel has been
recorded in New South "Wales :—
County Arrawatta—Severn River.
„ Bathurst—Bathurst District.
,, Clarke—Oban.
,, Gough—Yarrow Waterholes.
„ Hardinge—Tingha.
„ Murchison—Bingera.
,, Phillip—Cudgegong River.
„ Sandon—Uralla.
GARNET.
Crystalline system—Cubic.
Harduess—G'5—7'5.
Specific gravity—315—4'3.
Lustre—Yitreous to resinous.
Cleavage—Parallel to the faces of the rhombic dodecahedron, sometimes distinct.
Composition—Unisilicates
of various sesquioxides and protoxides. The sesquioxides being alumina,
iron, or chromium, and sometimes manganese. The protoxides are those of
iron, lime, magnesia, or manganese.
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