20 GEMS AND PRECIOUS STONES.
The
material of which they are principally composed is one of the most
plentiful of the constituents of the earth's crust. As silicate of
alumina in clay it exists in its most plentiful and useful form, while
in the ruby we find it as the most rare and costly of all the treasures
of the earth.
Corundum
does not appear to have been considered of much scientific interest
until the latter part of the eighteenth century, although it had been
used for dressing stones four thousand years before. It was, until late
years, invariably obtained from surface washings; but a remarkable
deposit having been found in Macon County, North Carolina, this mineral
has, since the year 1872, been systematically mined there. It occurs in
veins running through serpentine.
The
peculiarity of the corundums from this locality is that they exhibit in
some specimens several hues in the same stone. Some very large crystals
have been found in these veins; one weighing 312 1b. is in the cabinet
of Amherst College, U.S.A.
Nine
different varieties of corundum have been taken from these mine3 since
they were first opened, and the discovery is of the greatest
importance, as both rubies and sapphires were there found in situ in good quantities, and, according to Mr. Streeter, this is the only known instance of their being thus found.
Geological Formation.—Corundum
is found usually in beds of rivers, as water-worn crystals, and is
often associated with other precious stones, several of its varieties
being found in the diamondiferous drifts of this Colony. In India it is
principally found in the old crystalline or meta-morphic rocks, and it
is from the southern portion of that country that the greatest quantity
of corundum is obtained. (Our modern name corundum is derived from the
Hindoo word Kurand). Mr. Mallet has given an account of a
remarkable mine in India, where he states that above a mass of
por-phyiitic gneiss and hornblende rock, there lies a bed of corundum
several yards thick, and of areddish and purple grey colour; upon this
rests a bed of white and green jade, with purple corundum and other
minerals. At St. Gothard (Switzerland) it occurs of a red and blue
colour in dolomite, and near Mozzo, in Piedmont, in white compact
feldspar. In America it occurs in Maine, at Greenwood, crystallized in
mica-schist, with beryl and zircon, and at other localities in granular
limestone ; so that its distribution throughout the older rocks is well
marked, and it is not restricted to any one formation. It is sometimes
found in basalt, and is stated to so occur in this Colony. It is very
probable that good specimens of coloured corundums will eventually be
obtained from Central Queensland. I have seen some from that locality
changing in portions to blue, and of good size.
Coloured corundums are invariably dichroic, and are thus easy of deterÂmination, as will be shown under the variety ruby.
Friction excites electricity, and in polished specimens the electrical attraction continues for a long time.
Ordinary solvents have no action upon this mineral.
Corundums
crystallize principally in double hexagonal pyramids, that is, having
two pyramids, each with six faces, placed base to base. These pyramids
are more elongated than those of quartz crystals. A portion of the
prism may separate the pyramids.
We
will consider the word corundum to be appropriated by the common
varieties, having a grey, greenish or reddish colour, and which are
dull, and pass on to the gem varieties. The brown variety is known as Adamantine Spar.
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