Precious
opal is a gemstone that exhibits flashes of rainbow-like colors when
viewed from different  angles. The colors are created by an arrangement
of silica spheres within the opal. Common opal does not have this flash
of color. Opals
have a light or dark body color that may be either translucent or
transparent. The opals we see most often in jewelry have a milky-white
base with varying flashes of color. Black opals
are the most rare and expensive opals, with a body color that can range
from dark grey to black with fiery flashes of color. Precious fire
opals are another alternative. They have base colors ranging from
yellow to orange, and can be transparent to translucent with fiery
flashes.
Opal
hardness varies, but generally ranges from 5.5 to 6.5 on the Mohs
scale. Compare that to diamond, which rated at 10 is the hardest gem. Do
not use harsh cleansers on your opals and do not clean opals with
ultrasonic cleaners. Never rub oil on an opal, because although it
makes the fire brighter it can damage the opal. Some jewelers recommend
you periodically soak opals in water to help them reabsorb lost
moisture. Opal Mining: Australia
is the leading source for opals. but a great number of fire opals are
found in Mexico. Other sources of precious opals include Africa, the
United States, Brazil, areas of the former Soviet Union and Nicaragua.  Opal
doublets and triplets are composite stones that are made by positioning
a thin layer of opal on top of a chunk of glass or less expensive
gemstone. To find a composite, inspect the sides of an opal, looking
for indications that it's really a stack of layers. You can also check
the stone's base to see if it matches the top. If it doesn't it might
be a composite.
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