Opal is a non crystalline form of the mineral silica which, despite its
amorphous structure, displays an amazing degree of internal organization.
Opal is related to its more commonly found but highly crystalline cousins
quartz and agate, and is formed from amorphous "balls" or lumps" of silica
rather that from ordered, naturally faceted crystals.
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When dinosaurs
roamed the earth, the process of creation forged a unique gem which
somehow appeared to entrap fire and light in a myriad of colors and thus
OPAL was born.
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Black/Dark
Opal
Found mainly at Lightning Ridge in NSW and some areas in South
Australia, this magnificent variety in gem quality is arguably the
most valuable. Ablaze with colour, its dark appearance distinguishes
it from Light Opal.
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Boulder
Opal
Found in South Western and Central Queensland, this beautiful
variety of Opal is similar in appearance to Black Opal and forms
naturally within ironstone "Boulder" which is cut to form part of
the gemstone. |
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Light Opal
Found mainly in South Australia at Coober Pedy, Mintablie and
Andamooka. With brilliant reds, blues and greens intermingled in a
white or light background.
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Doublet and
Triplet
A thin slice of colorful Light Opal is laminated onto a layer of
dark material (usually Opal potch) to form a Doublet. The addition
of a clear dome of crystal (where the Light Opal is sandwiched
between the clear dome and the layer of dark material) makes it a
Triplet. The capping gives the Opal layer protection. Both varieties
are water sensitive and hence stones should not be immersed. |
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The chemical
composition of opal is SiO2H2O, silicon dioxide combined with water (an opal
stone may contain up to 30% water.) The silicate minerals in the stone add
to its weight, giving it a specific gravity ranging from 1.98 to 2.5 times
that of pure water. Opal's scratch hardness is measured at 6.0 to 6.5 on the
Mohs' scale, similar in hardness to quartz, a little more than halfway
between the hardness of talc and diamond.
Most opal is more than 60 million years old and generally dates back to the
Cretaceous period when dinosaurs roamed the earth.
It is found near the earth's surface in areas where ancient geothermal hot
springs once flowed. The minerals bubbled up from beneath the surface of the
earth and slowly, over the centuries, lined the walls of cracks, vents and
underground cavities in the bedrock. Most opal is found where geothermal hot
springs dried up during seasonal periods of rainfall and extended dry
periods.
More than 90% of the world's quality gem opals come from Southern Australia,
although it can be found in other parts of the world such as Brazil, Mexico,
Czechoslovakia and Nevada.
Opal has a
variety of poor gemstone characteristics, softness, dehydration, cracking,
physical weakness, and sensitivity to heat. It also shows one of the best
spectral displays of any gemstone, hence its value. It is made up of layers
of precipitated silica spheres in a jelly-like water mass, and the ordering
of the spheres sometimes produce a diffraction grating, that creates a play
of rainbow sparkling light from within the stone.
It is classified by its back ground color, the particular colors and
intensity of color display, and its size. Stones that are predominantly
white or light blue are the most common, and those that contain reds,
oranges, and violets are considered more desirable. Blue and green are very
common in most precious opal. Black opal, opal containing a predominantly
dark background (dark-gray to blue-black) is the rarest, and most desired of
all opals. When it contains reds and oranges it brings even a higher value.
It may be priced right up with the top gemstones (diamond, emerald, and
ruby). The very best black opal came from Lightening Ridge. Australia and
small amounts till reach the market today, but there have been no major
finds in many years.
How are Opals valued?
Opal is usually sold by carat weight, (1 carat = 200 milligrams). Valuing
Opal is difficult and the following key factors are considered the most
relevant in any appraisal:
Body Tone/Color -
Black or dark opal is considered more valuable than white or light opal.
Technically opals are rated according to their
Body Tone "N" Value from 9 (black) down to 1
(light transparent)
Play of Color - The
coverage of the play of colors is important. There should be no patches of
inferior colourless opal. Colours in order of value in Australian Opal are:
1. Red
2. Violet
3. Orange
4. Yellow
5. Green
6. Blue
Intensity of Color -
The brilliance and pattern of the colors present and the number of colors
present. The intensity and clarity of the spectrum colors is very
influential.
Soundness - Flaws
and cracks will obviously diminish the value of an opal. Inclusions of
minerals may also lower the value.
Shape and Polish -
The quality of the work performed by the opal cutter can significantly
affect the value of the Opal. It should be polished so that no scratches or
imperfections are visible to the eye.
Appeal - Because
opal is so unusual the appeal of a particular opal weighs more heavily than
almost any other consideration. This is a double-edged sword. If it appeals
to you as the buyer you will be willing to buy it. If it appeals to the
seller you may have to pay extra to sever their emotional attachment
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Crystal Opal
- is a translucent form of opal with vibrant colors, which play with the
light. Crystal can be found within all opal varieties.
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 Yowah
Opal (and Yowah Nuts) - is only found in
the Yowah and surrounding opal fields in southern Queensland. Several
forms and qualities of opal will often be found in the same small piece
of host iron-stone. When cut and polished they sometimes take on an
appearance of aboriginal art work. |
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