A Hand-book of Precious StonesRothschild, Meyer D.
Science
A Hand-book of Precious Stones
Rothschild, Meyer D.
Precious stones
Vermeille is a name given to the orange-red almandine, Cape garnet to
the bright red-yellow variety, Cape ruby to the pyrope, and American
ruby to the blood-red kind found in New Mexico, Montana, and Arizona.
Carbuncle is a term applied _to all_ garnets cut with a smooth rounding
top, sometimes called, after the French, cabochon.
Uwarowite or lime-chrome garnet is one of the rarest and most beautiful
of the garnet group.
The color of this stone is emerald-green, hardness 7.5, and specific
gravity 3.41 to 3.52. Uwarowites are found near Bissersk in the Urals
of Russia, but rarely in specimens of sufficient size to cut into gems.
This garnet is heavier and harder than the true emerald.
Demantoide or Bobrowska garnet is a soft garnet, olive-green to brown
and blackish-green in color, sometimes light green. It is found in
the Bobrowska River in the Urals. The specific gravity is 3.85,
and hardness about 6, its softness making it undesirable for many
ornaments. Before the blow-pipe it fuses into a black bead.
These garnets are often sold as olivines; they are heavier than
olivines and softer.
Demantoide is composed of:
Silica 35.44
Lime 32.85
Sesquioxide of iron 32.85
Magnesia .20
TOPAZ.
Topaz belongs to the rhombic system of crystallization. Its cleavage
is basal and perfect, fracture uneven, hardness 8, scratching quartz
distinctly, specific gravity 3.4 to 3.6, lustre vitreous, refraction
double, and colors ranging from colorless or white to bluish-white,
light blue, wine-yellow, straw-yellow, golden-yellow, greenish- and
pale-red to pink.
Topaz becomes electric from rubbing or pressure, and retains
electricity for twenty-four hours. Before the blow-pipe topaz partly
loses color, but does not melt, and with borax it fuses slowly to a
white bead.
Topaz is partially attacked by sulphuric acid, and dissolves in salts
of phosphorus.
The composition of topaz is:
Silicon 15.05
Aluminium 30.02
Oxygen 36.08
Fluorine 17.05
Goutte d’eau or colorless topaz, sometimes called “slaves diamond,”
Siberian or bluish-white, Brazilian or golden to reddish-yellow, Saxony
or pale-wine yellow, Brazilian ruby or pink, Brazilian sapphire or
light blue, and aquamarine or greenish, are the various commercial
names for topaz.
Most of the Brazilian rubies or pink topazes are produced by heating
the reddish or dark-yellow variety, either in a crucible or by
enveloping the stone in German tinder and setting fire to the tinder.
If heated too much, the stone is apt to become colorless, and if
suddenly cooled it may crack.
Colorless or white topaz takes a very high polish, and is wonderfully
clear and transparent.
The great Portuguese diamond, “The Braganza,” of about 1,680 carats, is
supposed to be a white topaz.
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