- While each opal is unique,
there are some basic characteristics which can be used to determine an opal's
- These characteristics are,
more or less in order of importance, type, brightness of fire, base color,
fire color, fire pattern, rarity, cut and consistency of fire.
- There are seven main types
of opal and opal substitutes; solid opal, boulder opal, matrix opal, treated
(dyed) opal, assembled opal (doublets and triplets), man-made (synthetic)
opal and opal stimulants.
- Solid opals can be all precious
opal or a combination of precious and common (potch) opal.
- Assembled opals may have backs
of common opal, ironstone, jade, or glass, etc.
- Boulder opal is a natural combination
of precious opal and ironstone or other parent rock. Boulder opal consists
of a line of precious opal with an ironstone back, while boulder matrix is
ironstone with precious opal mixed through it.
- Treated (dyed) matrix opal
has a satin finish, looks porous under a loupe, and/or has a distinctive matrix
pattern with an edge.
- Triplets can be easily identified
by the clear quartz top to the stone.
- Doublets can be identified
by the straightness of the line of color and the black paint and glued at
the joint visible with a loupe.
- Type is the single most important
determinant of opal value.
- Synthetic opals have extraordinarily
bight fire, and overly regular pattern with roundish globular edges, and distinctive
columns of fire when viewed from the side.
- Imitation opals are often plastic.
In most cases the flashes of color do not look like natural opal.
- Dyed matrix has typical matrix
characteristics: satin finish, looks porous, and has a distinctive matrix
- There are two characteristics
which make up the term base color; color and clarity.
- The term black opal is frequently
misunderstood. It should be used only to refer to dark base color stones meeting
the criteria of the Lightning Ridge Miners Association Tone Scale.
- Base color (tone) is determined
with reference to the Lightning Ridge Miners Association Tone Scale.
- Solid opals and boulder opals
are never referred to as "natural doublets."
- The dark base colors are referred
to as black, semi-black and gray. In addition the rare red, orange, yellow,
blue, green, and brown base colors.
- Clarity refers to the degree
to which you can see through an opal. The range is from transparent (referred
to as crystal), through translucent (referred to as semi-crystal), to opaque
(with no special terminology).
- The brightness of the fire
bouncing back from an opal is a major factor of its value, the brighter the
- Lighting has a major effect
on the appearance of brightness.
- Moving the opal away from the
light gives you an idea of how well the brightness holds as lighting changes.
A truly exceptional opal is one that is brilliant (level 5 brightness) in
and out of the light.
- To characterize the fire colors
found in an opal, the most dominate color is referred to first. Other colors
which contribute significantly to the look of the stone are added. Thus, an
opal which shows a dominate orange and supporting red would be referred to
as having orange-red fire color.
opal would be considered a multicolor stone only if it has at least three
strong fire colors.
- While every opal has a unique
pattern, there are seven categories of patterns that all opal fit within:
Pinfire, Flashfire, Broad Flashfire, Rolling flashfire, Harlequin, Rare patterns
and Picture stones.
- Over 90% of all opals have
patterns which fit into the flashfire or broad flashfire categories.
- The term Harlequin is frequently
misunderstood and misused. It refers only to regular square or angular patches
of fire closely set together.
- Cut refers to the shaping of
the opal. Most opals are cut into cabochons.
- The most popular shape of an
opal cabochon is an oval. When ovals are of standard dimensions, they are
referred to as calibrated opals.
- Poorly cut shapes include excessively
fat stones, lack of a shoulder for setting, thin edges, misshapen dome, excessively
thin stones, and the lack of a bevel on the bottom edge of the opal.
- Lightning Ridge stones are
cut with a heavy belly to add weight to the stone. This belly has already
been adjusted of the opal's price per carat.
- Good cut also requires that
the opal should be free from scratches visible to the naked eye and that it
have a smooth surface.