Composite vs Assembled Opal

Composite opal
These stones are partly made up of opal and partly of the rock which the opal was found in. The rock may be ironstone, quartzite, sandstone or another matrix. This type also includes pseudomorphs in which the opal has replaced another substance. Composite opal consists of a single mass and should not be confused with assembled stones.

Assembled stones normally consist of two parts (doublets) or three parts (triplets) cemented or fused together for providing a better wearing surface; to obtain a large stone from two smaller pieces of natural material; to produce a stone of apparently better color and appearance; for the purpose of supplying a rigid backing or support for fragile materials.

Doublet
Opal doublets consist of a thin slice of precious opal cemented onto a plastic, black onyx, glass or potch (common opal) back. To lower the cost, the top surface is normally flat. Better (and more expensive) doublets are made with thicker pieces of opal, and the surface is domed. Black cement is used to give a dark background for the translucent opal. This gives the appearance of an expensive black opal. Crystal opal makes the best doublets and gray opal the worst. The identification of doublets is not normally difficult, but one must be careful of opal doublets with a potch backing because some solid opals also have a potch backing.

Triplet
As the name implies, opal triplets consist of three parts. The middle section is a thin slice of precious opal, the back is the same as on a doublet, and the top is usually made of rock crystal quartz or another transparent colorless material. This top piece gives the stone greater durability and also magnifies the play of color, thus increasing the beauty. In general, triplets are worth more than doublets. Clear resin is used to cement the quartz top to the opal, and dark resin to cement the opal to the back. Synthetic opal (Gilson) has also been used in place of natural opal for making opal doublets and triplets.

Opal doublets and triplets are easily identified. Unlike other assembled stones, they are not intended to deceive the buyer. They are produced in order to make use or otherwise useless material, and provide the beauty of a fine solid opal at a fraction of the price.