‘Play of Colour’ is a unique visual phenomenon which sets precious Opal apart from all other gemstones. Also known as 'Fire'; An Opal may display one or more, and sometimes all of the spectral colours. These colours are seen within the grains of a pattern. As the stone is viewed from different directions, rotated and tilted, the colours of each grain may change or disappear.
Types & Varieties of Natural Australian Opal
TYPE 1 - NATURAL; 'Light, Dark & Black Opal'
Black Opal has a blue-black to charcoal 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. Black Opal is found as what the miners call ‘Nobbies’, these are fossil replacements of corals or sponges. During its formation, the replacement of organic material by Silica resulted in carbonaceous material or impurities like titanium impregnating the mineral structure giving Black Opal its body colour.
(Variety: Black *(pictured), Black Crystal)
Dark Opal has a smokey to dark grey body tone and may be found at Lightning Ridge NSW. In South Australia Mintabie is famous for Semi Black and Dark Crystal Opal, as are certain fields at Andamooka and Coober Pedy.
(Variety: Semi-Black, Dark Crystal *(pictured), Dark Grey, Smokey Crystal, Smokey Jelly)
(Variety: White *(photo), Grey, Crystal, Jelly)
Body Tone Scale
TYPE 2- NATURAL 'Boulder Opal'
Boulder Opal is found in south west and central Queensland and can be light or dark in appearance. Boulder Opal forms as veins within ironstone concretions and is presented as a solid (one piece) since it is naturally attached to the host rock. Boulder Opal may display the same darkness and brilliance as gem Black Opal yet it is more affordable and is generally sold at approximately one third of the Black Opal carat price. Boulder Opal is very durable due to the ironstone backing the opal forms on, which the cutter retains and polishes during the lapidary process.
The water content in Boulder Opal is very low, it hardly ever cracks or crazes as it ages and is considered the most stable type of natural Opal by many experts.
Variety of Boulder Opals:
1) Boulder Pipe
2 ) Black Boulder (green-orange fire)
3) Gem Boulder (split pair)
4) Black Boulder (red fire)
5) Yowah Nut Opal (split-sawn pair)
TYPE 3 - NATURAL 'Matrix Opal'
Matrix Opal - is silicified sandstone or ironstone which is intimately diffused with Opal as infillings of pores or holes or between grains of the host rock in which it was formed.
Opal is sometimes encased in or intertwined with the Boulder concretion creating ‘Boulder Matrix’, ‘Yowah Nuts’ and other interesting textures sometimes referred to as ‘picture’ or ‘fun stones’.
|1) Light Boulder Yowah Kernel||2) Boulder Opal Matrix|
Modified & Man-Made Opal
TREATED OR ENHANCED STONES
Treated Matrix - Although some rare examples may actually resemble Black Opal in their natural state, the vast majority of 'Andamooka Matrix' from South Australia, and 'Fairystone' (Sandstone Opal from Queensland) are artificially stained or blackened to enhance their appearance and may also be 'impregnated' with plastic stabilizers to improve the finish.
Heat & Acid Treatment - Since the 1950's miners have treated the rock in sugar solution then carbonizing it by gently heating in concentrated sulphuric acid. Carbon is deposited into its pores by a process involving soaking in various organic solutions and applying heat up to 500 degrees Celsius.
|1)Treated Andamooka Matrix|
The rough material is fairly massive in occurrence and has been found across all the Opal fields. In the rough state it is a muddy-grey to orange porous rock and lacks fire, once treated this stone may be startling in quality. This versatile and relatively inexpensive material has applications in jewellery, carvings, clock faces, mosaics, inlays in furniture, tiles, paint finishes etc.
Reputable Gem dealers have a duty to disclose such modifications to the buyer.
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.
However some Boulder Doublets may display an irregular or wavy join line as a mixture of powdered ironstone and resin may be used to imitate the irregularity of natural Boulder Opal.
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.
In 1980 Kyocera Corporation of Japan began producing Inamori black and white synthetic Opal. Russian manufacturers are also producing synthetics.
Made simply by imbedding iridescent foil in opalescent (milky) glass Opal imitations date back to ancient times.
These are easily detected on sight.
Slocum Stone made of glass (SG 2.45, RI 1.49-1.515) was the best Opal imitation available until 1978 when the Japanese produced plasic simulants that are somewhat more convincing and similar to Gilson to look at. This material has the same refractive index (RI 1.45) as natural Opal however it is lower in density (SG<1.58), often highly porous with a waxy texture.
www.bolda.com.au - 100% NATURALLY OCCURRING AUSTRALIAN GEMS & JEWELLERY
Sources & Image Credits:
WITHIN THE STONE, Bill Atkinson, 2004. (Photo.1: Phenomenal - Play of Colour)
THE Opaline COLLECTION, Exceptional White, Boulder & Black Opals
HANDBOOK OF GEMSTONE IDENTIFICATION, Richard T. Liddicoat, Jr., 1990
NCJV OPAL GRADING & PRICING WORKSHOP, Ralph Pownall, 2011.
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