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PRECIOUS STONES.
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from
a vast number of minute parallel surfaces. Amongst other localities are
the Carboniferous volcanic rocks of Fifeshire, and in veins in the
"plate" or lower Carboniferous shales of Alston Moor, in Cumberland. It
is nearly always found in narrow veins, and usually completely fills
them. It may be cut en cabochon, to show the chatoyant effect, but its softness renders it very prone to damage.
Of
the massive varieties Fire-marble, or Lumachello, may be noticed. It is
a fossiliferous marble, with a beautiful iridescence of red, green, and
blue colours, almost comparable to the Opal. It is chiefly found in
the lead mines of Bleiberg, in Carinthia.
What
was known to Pliny as the Alabastrites was a form of carbonate of
calcium, deposited from water, in layers. It might be either
stalactitic (formed by lime-bearing water dripping from the roof of a
cave), or stalagmitic (formed by the water dripping on the floor). When
it showed pale creamy bands it was known as Onyx, from its resembling
the finger-nails of the " well-bred person." The term Alabaster was
used on account of this substance frequently forming the material of
the Alabastra or ointment jars (King). Much of this Alabaster was found
near Thebes, and has thus come to be known as Egyptian Alabaster, to
distinguish it from the compact Gypsum, which is also called Alabaster.
Much of this stalactitic material, when first formed, is probably
Aragonite, a rhombic carbonate of calcium which tends to pass into
Calcite. Alabaster is translucent, and is capable of taking a very fine
polish, but on account of its softness it is easily scratched.
"Gibraltar Stone" and " Mexican Onyx" are named from the localities
where they are found.
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