Classifying Opals

Unlike solid opal, boulder opal is often cut to include some of the parent rock.

There are three main factors in the description of an opal: base color, transparency, and type. The color of an opal can be a point of confusion since most opals display several different colors. There are two color characteristics in opals; the background color and the color associated with the fire. It is the background color that is being referred to when an opal is described as a black opal or a white opal. If you ignore the fire in a black opal, the background color is black. Due to its complex non-crystalline composition, opals interact with their environment in a variety of ways to create variety of background colors. The color can originate from a substance that is naturally associated with the opal or it may originate from the opal itself. Opal with fire can occur in a variety of background colors. Australia produces a lot of white and semi-clear material, and is most famous for its black opals.

When an opal is described as “crystal” or “semi-crystal”, it is the transparency that is being described. This terminology can be confusing because opals do not have a crystalline composition. A “crystal” opal is an opal that is transparent (no base color). A “semi-crystal” opal is an opal that is semi-transparent (translucent).

There are several different types of opals. Below are some of these types:

Matrix opal is mixed in with the parent rock and usually can't be cut into solid opal.

Precious Opal: opals that have any amount of color flashes. Opal without any color is potch opal (common opal).

Potch Opal: sometimes referred to as “common opal”, potch is opal with no color.

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).

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.

Matrix Opal: When the opal material is mixed with the parent rock, it is called matrix opal.

Treated (Dyed) Opal: Sometimes a light or clear opal is dyed to imitate a black opal.

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.

Doublet: 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.

Triplet: 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.

Man-Made Opal: This is an opal grown in a laboratory.

Imitation Opal: These are made of non-opal materials (plastic, glass, etc.). They are meant to imitate the look of an opal.

Fire Opal: often confused with “fire” which is in the opal. The flashes of color are called “fire”, but the term “fire opal” is any opal with an orange background. A fire opal may have red flashes, blue flashes, or hardly any flashes at all.

Gem Opal: while there is no precise definition, gem opals typically refer to high quality opals with an abundance of fire.