Opal Dictionary
Assembled Opal
These Opals have been glued together.   A doublet is a two-piece stone made by gluing a piece of precious Opal to a piece of common opal or a non-opal material.  A triplet is a three-part stone.
Background Color
The color of the Opal when the fire color is disregarded.   White opals are the most common opals and typically less valuable than the rarer black Opals.  Black Opals are the most sought after background color.
Black Opal
Opals with a black or nearly black background color.   These rare and highly prized Opals are primarily found in the Lightning Ridge mining area in Australia.
Blind Angles
When an Opal is rotated, if the color disappears at any angle, that angle is a blind angle.   Many blind angles will depreciate the value of the Opal no matter how bright the fire is from one particular angle.   The worst blind angle is when the viewer is looking directly at the front of the Opal.
Boulder Opal
When Opal is found in thin seams, sometime an Opal is cut to include part of the parent rock.   The color of the parent material influences the base color of the Opal.   These Opals are called “boulder” Opals.
Blind Spots
When an Opal is stationary, the areas of the Opal not displaying colors are blind spots.  An Opal with a smaller percentage of blind spots will generally be more valuable than one with a larger percentage.   A valuable Opal will have fire that covers most of the Opal from all angles.
Crazed Opal
Cracks that develop in unstable Opals due to the water in the Opal drying too quickly.
Crystal Opal
Any Opal with a transparent background color.   If you can see through the Opal, then it is a crystal Opal.   This can be a confusing term, because Opals are not crystalline gems.
If rough Opal is too thin to be cut into a gem, sometimes they are glued to a darker stone to provide a black background resembling a black Opal.   The background may be dark potch, ironstone or any other darker material.   These are substantially less valuable than solid Opals.
The colored flashes associated with precious Opal.
Fire Opal
Often confused with “fire” which refers to the colored flashes.  "Fire Opal” is an Opal with an orange background.  A fire Opal may have red flashes, blue flashes, or hardly any flashes at all.
Gem Quality Opals
While there is no precise definition, gem Opals typically refer to high quality Opals with an abundance of fire.
Potch Opal
Sometimes referred to as “Common Opal”, potch is Opal with no color.
Imitation Opal
These are made of non-opal materials (plastic, glass, etc.).   They are meant to imitate the look of an Opal.
Man-Made (Synthetic) Opal
These are composed of the same substance as natural Opals, but they are grown in a laboratory.  Their value is substantially less.
Matrix Opal
When the Opal material is mixed with the parent rock, it is called matrix Opal.
Precious Opal
Opals that have any amount of color flashes.   Opal without any color is potch Opal (common Opal).
Semi-Crystal Opal
Any Opal with a semi-transparent (translucent) background color.   This can be a confusing term, because Opals are not crystalline gems.
Solid Opal
Most Opals are solid.   Solid Opals are Opals with no non-opal material attached.   The Opal may contain inclusions or a small amount of parent material or non-precious Opal (potch).
Stable Opal
Opal that is not very susceptible to cracking due to its low water content.
Treated (Dyed) Opal
Sometimes a light or clear Opal is dyed to imitate a black Opal.
Made by gluing a thin layer of Opal onto a piece of potch, ironstone, or some other dark substance.   A piece of glass or quartz is then glued on top of the layer of Opal.   These are substantially less valuable than solid Opals.
Unstable Opal
Opal that is very susceptible to cracking due to its high water content.
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