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domingo, 26 de março de 2017
Types of Opal
Types of Opal
Opal is a hydrated amorphous form of silica; its water content may
range from 3 to 21% by weight, but is usually between 6 and 10%. Because
of its amorphous character, it is classed as a mineraloid, unlike the
other crystalline forms of silica, which are classed as minerals. It is
deposited at a relatively low temperature and may occur in the fissures
of almost any kind of rock, being most commonly found with limonite,
sandstone, rhyolite, marl, and basalt.
Opal is the national gemstone of Australia. Australian opal has often
been cited as accounting for 95-97% of the world’s supply of precious
opal, with the state of South Australia accounting for 80% of the
world’s supply. Recent data suggests that the world supply of precious
opal may have changed. In 2012, Ethiopian opal production was estimated
to be 14,000 kg (31,000 lb) by the United States Geological Survey. USGS
data from the same period (2012), reveals that Australian opal
production to be $41 million. Because of the units of measurement, it is
not possible to directly compare Australian and Ethiopian opal
production, but these data and others suggest that the traditional
percentages given for Australian opal production may be overstated. Yet,
the validity of data in the USGS report appears to conflict with that
of Laurs and others and Mesfin, who estimated the 2012 Ethiopian opal
output (from Wegal Tena) to be only 750 kg (1,650 lb).
The internal structure of precious opal makes it diffract light;
depending on the conditions in which it formed, it can take on many
colors. Precious opal ranges from clear through white, gray, red,
orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown,
and black. Of these hues, the reds against black are the most rare,
whereas white and greens are the most common. It varies in optical
density from opaque to semitransparent.
Common opal, called “potch” by miners, does not show the display of color exhibited in precious opal.
11.0 x 7.0 x 2.5 mm
Also known as ‘milky opal’, white opal features light white body
tones, and is mined in South Australia. White opal is more common and
because of its body tone, generally does not show the colour as well as
black opal. Nevertheless, white opals can still be absolutely
magnificent in colour if a good quality stone is found.
7.75 ct. Lightning Ridge Crystal Opal Credit: Mardon Jewelers
Crystal opal is any of the above kind of opal which has a transparent
or semi-transparent body tone – i.e. you can see through the stone.
Crystal opal can have a dark or light body tone, leading to the terms
“black crystal opal” and “white crystal opal”.
Is a term sometimes mistakenly and improperly used to refer to fire
opals, as well as a type of transparent to semitransparent type milky
quartz from Madagascar which displays an asterism, or star effect, when
cut properly. However, the true girasol opal is a type of hyalite opal
that exhibits a bluish glow or sheen that follows the light source
around. It is not a play of color as seen in precious opal, but rather
an effect from microscopic inclusions. It is also sometimes referred to
as water opal, too, when it is from Mexico. The two most notable
locations of this type of opal are Oregon and Mexico.
Peruvian Opal (also called Blue Opal)
Is a semiopaque to opaque blue-green stone found in Peru, which is
often cut to include the matrix in the more opaque stones. It does not
display pleochroism. Blue opal also comes from Oregon in the Owhyee
region, as well as from Nevada around Virgin Valley.
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