What is an Opal?

National Gemstone of Australia

'Play of Colour’ is a unique visual phenomenon which sets precious Opal apart from all other gemstones. An Opal may display one or more, and sometimes all of the 7 spectral colours. The colours are seen within the grains of a pattern. As the stone is viewed under different lighting conditions or from different directions, the colours of each grain may change as may its shape, revealing a lively moving pattern.

 

Three Varieties of Natural Australian Opal
Each variety is a classification determined by background or body colour and may include several types as outlined.

White Opal has a body tone ranging from milky white to translucent. Found at most fields but mainly at Coober Pedy and in South Australia. Crystal Opal tends to be transparent with bright colour flashes suspended in its midst. White Opal, with its lighter to whitish body colour, gives the full colour array in a more opaque background.

Black Opal has a blue-black to dark grey body tone and is mainly found at Lightning Ridge in New South Wales. The dark background serves to highlight the colour-play of dramatic spectral flashes. Fine examples of this variety are the most expensive per carat and rival diamonds in price.

Boulder Opal is found in south western Queensland and can be light or dark in appearance, it forms naturally as veins within ironstone boulder which is cut to form part of the gemstone. Boulder Opal may display the same darkness and brilliance as gem Black Opal yet it is more affordable. is very durable due to the ironstone backing the opal forms on,and is generally sold at a fraction of the Black Opal carat price.

Many unique examples of Boulder opal are encased in or intertwined with the Boulder mother stone creating ‘matrix’, ‘Yowah nuts’ and other interesting textures known as ‘picture’ or ‘fun stones’.

Boulder Opal which the cutter retains and polishes during the lapidary process.

The water content in Boulder Opal is very low, it almost never cracks or crazes as it ages and is considered the most stable variety by many experts.

(Types: light boulder, black boulder *(photo#2), Yowah nut *(photo#1), boulder pipe *(photo#3), boulder matrix)

 

Man-made Opal stones

Composites - Doublets & Triplets are made using a slice of Light Opal and adhering it to a backing of black potch, plastic or boulder.
These are not investment quality stones but generally cheap products aimed as souvenirs. They lack durability and exposure to moisture can result in the opal veneer separating from its’ backing.

Doublets are made by gluing a slice of Opal to a black backing which enhances the reflection of colours from the Opal.
 Composites are detectable if the stones’ edges are examined, definite layers can be seen, under magnification. But some Boulder Doublets may display an irregular join line. If a pin is heated with a naked flame and then pushed into the join line, it should penetrate easily as the backing which looks like ironstone is actually powdered ironstone in an epoxy resin.  

Synthetic or ‘Gilson’ black and white Opals have all of the gemmological properties of natural Opal. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974.
However there are several points of detection for this man made product;
•    Freedom of inclusions like potch or sand, and a stone which looks too perfect.
•    Gilson White Opal does not fluoresce under UV light unlike most natural white Opal.  
•     Very regular play of colour, with colours always arranged in uniform layers on vertical columns may arouse suspicion. Under magnification this has the effect of resembling a ‘snake or lizard skin’ pattern.

Imitation Opals are made simply by imbedding iridescent foil in opalescent (milky) glass. These are easily detected on sight.
 
But there are also Japanese simulants that are somewhat more convincing and similar to Gilson to look at, but they contain substances not found in natural Opal, such as plastic stabilizers.
These simulants are lower in density and often highly porous and have a waxy texture.

Treated Opals include Andamooka Matrix and Sandstone Opal from Queensland which is often stained or blackened to resemble black opal and may also be treated with plastic stabilizers to improve the finish. These stones are always opaque and may be detected by their porosity, lower hardness and density (Specific Gravity).