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Your
Field Guide to Opal Fact and Folklore |
| Opal is Australia's national gemstone. Ninety-six percent of the world's
gem quality opal is mined in Australia! The other four percent comes from Mexico, Central
America, the United States and parts of Europe, including 2000 year old mines in the
eastern parts of the old Czechoslovakia. |
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| Most opal is 50-65 million years old, dating back to
the Cretaceous period when dinosaurs roamed the earth. Opal formed as silica from
decomposing rocks mixed with ground water to form a kind of silica gel, which collected
and hardened in underground cavities and fissures. |
| Opal's chemical formula is SiO2 .nH2O, indicating
its make-up of silicon and water. There are two different types of opal, common and
precious. The formation of the silica particles determines the type and the value of the
stones. |
| In precious opal, the minute spheres of silica
particles are packed in regular rows and layers. When light hits this array it is
diffracted (or split), creating iridescent flashes of colour. |
| "Black Opal" is primarily
mined in and around the Lightning Ridge region of Northern New South Wales (NSW). It is the rarest and
therefore the most valuable variety of opal. Fine quality black opals can sell for more
per carat than diamonds. The term Black Opal is somewhat misleading as the opal is not
'black' in colour, but has a very dark base body tone, which enhances the brilliant
colours. |
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| "Boulder Opal" occurs
spasmodically within a belt of Cretaceous sedimentary rocks from the New South Wales'
border to about 1000 km north-west in Queensland (the "Boulder belt").
It consists of opal on a dark ironstone (the host rock). The opal occurs as a thin uneven
layer naturally adhering to the ironstone. Some ironstone may be visible in the surface
when cut. However, a Boulder Opal which has a full face with no ironstone intrusions is
said to be 'clean-faced' and commands a higher price. Queenland Boulder Opal accounts for
just 10 per cent of the world market, and because of its brilliance and rarity can fetch
$1000 a carat in the rough. |
Boulder opals are graded into specific groups, defined by the Australian
Gemmological Association:
- Boulder black opal,
- Boulder crystal opal,
- Boulder light opal,
- Boulder matrix opal,
- Yowah nuts,
- Fun stones,
- Boulder splits,
- Seam and vein opal,
- Sandstone opal,
- Pipe opal, and
- Wood opal.
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| "Light Opal" is the most common type
of opal found and is mined primarily in South Australia (SA). 'Light' opal refers
to both the crystal type opal (translucent) and the milky opal (opaque). As it is the most
common variety, it is generally the least valuable, however, gem quality Light Opal can
exceed the value of some Black and Boulder Opals. |
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| "Common Opal", or
"Potch", is found all over the world. This version is usually amber coloured and
valueless. |
| Brilliance or lustre is the intensity
of colour play within an opal and is one of the main factors determining its value.
However, every person sees opal in their own way, and an inexpensive stone can have great
personal appeal. You should buy what you like. |
| To view an opal correctly, stand with your back to a
light source. Changing the direction of the light source will cause the light to be split
though different voids in the array, and different colours will be visible. |
| Contrary to popular belief, Australian opal is not porous and will not
absorb contaminants such as perspiration or moisture. The water locked in an opal's tiny
voids is impregnable and will not escape unless heated to extreme temperatures, so soaking
in water or washing will not change your opal in any way. Opal's scratch hardness is
measured at 6.5 on the moh scale, and is similar in hardness to quartz. |
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