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Brilliance: The overall brightness and
intensity of the play-of-colour. The more brilliant the flashes of
colour, the more expensive it becomes. Examine brilliance both under a
consistent light source and away from it. Stones that maintain their
brightness away from bright light are more highly valued. Brilliance is
one of the most important value factors. Play-of-colour
(play-of-color): The combination of color and the dominant color are both
important. Intense red is the most rare and most prized color. In
terms of rarity and value, red is followed by orange, green and blue (blue is
the most plentiful color in opals). Prices vary between opal dealers who
place different values on color combinations. Any type of play-of-color
is desirable if the colors are intense and not dull when viewed face up. Color
Pattern: The diffracted colors in opals are displayed in various patterns.
Pinfire: small pin-point color specks;
Flashfire: larger splashes of color, usually irregular in shape;
Broad flashfire: sheets of color normally covering a large
section or all of the opal's surface;
Harlequin: square or angular patches of color set close together
like a mosaic;
Fancy Patterns and Picture Stones: unusual patterns that
form pictures or resemble flowers, cat's eye, Chinese symbols, etc.
Pinfire and small type patterns are generally less valuable than broad
patterns or large flashes. Harlequin and distinctive fancy patterns are
especially valuable. Transparency:
The higher the transparency in light opal and fire opal, the more valuable the
stone. For black opal, the opaque, blacker stones are usually more
valuable than those with more transparency. Shape:
A well formed oval, round or square opal is more expensive because of the
greater demand, it is easier to set into jewelry and less expensive opal is
lost when the stone is cut. Many jewellers and designers prefer freeforms,
and unusual freeforms already set may bring a greater price than a traditional
shape. Cut: Excessive weight on the
bottom and a thin, unsymmetrical profile can reduce the opal prices.
Domed cabochons tend to be more expensive than flat cuts. Size
and carat weight: Opals under one carat are generally priced less per
carat than larger ones. If a stone is unusually large, it may be worth
less per carat than opals more suited for jewelry. Boulder carats are
typically priced by size rather than carat weight. The larger the opal,
the greater the price. Imperfections:
Inclusions such as sand or gypsum visible to the eye on the top of an opal
decreases the value. The larger and more noticeable the imperfection, the
greater the affect on the value. Inclusions on the back of an opal have
little or no effect on price unless they impact on the structural integrity of
the stone. Cracks drastically reduce price. Crazing:
A common opal flaw is crazing, a thin network of fractures that resembles a
spider's web. If the crazing is deep, it has a great impact on value.
Opals with a high water content are most subject to crazing. There are
ways to conceal crazing, but no honest dealer would do that without disclosing
the fact. Check that your dealer offers a guarantee (usually one year)
against crazing. Dealers usually keep opals for a period of time to
ensure they are not subject to crazing. These guarantees are usually not
against cracking which are often the result of poor setting skills and abuse.
Opals occasionally crack or craze when cut and set, and with sudden
temperature changes. Over-heating can cause opals to turn white or
brownish and can cause the play-of-colour to disappear.
Opal is sometimes impregnated with oil, plastic or wax to improve the
play-of-colour and to prevent or disguise cracking. The plastic method is
stable, but oil and wax are not stable. There are various techniques for
creating the appearance of black opal, including smoke impregnation, backing
with black or coloured paint and treatment with dye, silver nitrate or sugar
carbonized by acid. Avoid repolishing or applying solvents to treated
opals. 
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Opals Buying Guide
