Monday, July 19, 2010

OPAL - Australia's National Gemstone

Based on several requests have decided to do a Blog on Opals. However this specific BLOG deals with Australian Opals. Many thanks must go to the Jewelers Association of Australia for their information and pictures. There are other major varieties of opals not covered here, Mexican Fire Opals (transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not show any play-of-color), Peruvian Opals (semi-opaque to opaque blue-green, green, blue or pink stone found in Peru which is often cut to include the matrix in the more opaque stones. They do not display pleochroism), to name a couple. Other varieties of opal can also be found in Slovakia, Brazil, Ethiopia and the Northwest and Southwestern United States with new finds now reported in China.
The Australian Opal. Nature has captured all the colors of the spectrum, displaying them in an infinite variety of shades, patterns, and brilliance and locked them into this rare stone. Opals lie dormant in the earth until a miner clips the edge and light reaches the gem exposing the most beautiful dancing displays of color. This ‘Play of Color’ is the basis for the opals beauty. It is unique in the world of gemstones. It originates from the breaking up of white light due to the three dimensional spherical shaped microstructure of the silica partials inherent in the opal. Similar color phenomena can be observed when oil lies on water. Opals occur where silica gel fills small fissures or voids in the earth. A large portion of Australia’s inland was once covered by a sea, leaving deposits and creating an environment suitable for the formation of the opal. As the cellulose of the tree wood decays, it leaves empty cells in the wood. As the silicon rich water seeps down into the void, it eventually dries out and leaves the silicon behind to form opals. Some of the water is retained by he silicon dioxide...which is why opals contain so much water. The silicon forms as small rows of round spheres that create the ‘Play-of-color’. Natural Opals are divided into categories to distinguish the location where mined and the specific variety of Opal.

BLACK OPAL is the most valuable and comes mainly from Lightning Ridge. High quality stones are very rare; this type is easily distinguished by the blackness of the base or background body tone.

BLACK CRYSTAL OPAL is mainly mined at Lightning Ridge. These black opals show a degree of transparency, the colors are often brilliant and can appear to come from within the depth of the gemstone. Good black crystal opals are extremely rare.

DARK OPAL comes from all the fields and is desirable because the colors are generally more brilliant, due to the dark background highlighting the colors. Good specimens of this type of opal mainly come from Mintabie and Lightning Ridge.

LIGHT OPAL is usually found at all opal fields but the bulk of the better material comes from the South Australian fields of Coober Pedie and Andamooka, although the first material was mined at White Cliffs. Some light opals are so light they are called White Opals (2nd picture above).

CRYSTAL OPAL embraces opals which are transparent or very translucent and in the better quality stones shows a distinct and very bright play-of-color. This type of opal is found in most Australian opal fields.

BOULDER OPAL is composed of opal naturally occurring on its host rock. It is mined predominantly in Queensland. It is easily identified because when cut, the host rock if left on the sides and/or back of the opal. Boulder Opal may be light, dark or black within the host rock. This type of opal in cabochon settings, has become very popular in the past few years, especially those stone that have the host rock all around the ‘window’ of opal in the center of the stone.

MATRIX OPAL comes in two types in Australia.

-BOULDER MATRIX OPAL is usually found in Queensland. The opal is intimately diffused with the host rock (usually ironstone so is magnetic), which is quite obvious in the presentation face of the stone.

-MATRIX OPAL from the Andamooka area has opal internally diffused throughout the host rock, a sandstone, which is often porous and can be treated, turning the material dark, resembling black opals.

NATURAL OPALS are the complete opal cut and polished as one piece. These can be very fragile and easily damaged when worn.

COMPOSITE NATURAL OPALS consist of a natural opal laminate manually cemented or attached to another material. There are three main types.

-DOUBLET OPAL is a composite of two pieces where a slice of natural opal is cemented to a base backing material.

-TRIPLET OPAL is a composition of three pieces where a thin slice of natural opal is cemented between a dark base material and a transparent top layer, usually of quartz or glass.

-MOSAIC CHIP OPAL is a composition of small flat or irregularly shaped pieces of natural opal cemented as a mosaic tile on a dark base material or encompassed into resin.


There are three (3) specific other Opal types that also come from Australia that I hope to gather further information on and be able to post a Blog in teh near future on them - Andamooka, Koroit and Yowah Opals. Check out the Opal Forum at

for informatin on these Opals.

SYNTHETIC and IMITATION OPAL has become a real problem worldwide, but not a problem in Australia. Australian law deals harshly with anyone making or selling synthetic or imitation opals and the few retailers who have them, will have them so marked and inform purchasers of this fact. You will see very few within Australia.

For our budding gemologist and geologist, opal is a mineraloid, it is an amorphous SIO2 - NH2O (hydrated silicon dioxide). The water content is usually between three and ten percent, but can be as high as 20%. It is a 6 on the Mohs scale of hardness. Opal has a RI: 1.44 - 1.46 -- Birefringence: None -- Optic Character: None -- Specific Gravity: 2.10 average but varies based on amount of water contained in the stone.

Opal is the traditional birthstone for October and the state gemstone of Nevada.

Caring for your Opals
As with Pearls, the number one care item for Opals is to never have them on when using hair spray or wear them when doing rough work.
Solid Opals -- Opal is a soft stone, so it is important to treat your opal carefully In order to avoid damaging it. Remove your opal jewelry if there is a chance it will be scratched or broken (i.e. working in the garden, moving furniture, etc.) Many people believe solid opals can be damaged by water - however, this only applies to doublets and triplets. Solid opals are fine in water. In fact, most precious opals contain about 5-6% water. As a result, opal may crack if subjected to very dry conditions or rapid changes in temperature. Try to avoid very high temperatures or low humidity extremes, such as boiling water or zero humidity bank vaults.
Doublets & Triplets -- Caring for doublets or triplets is a little different to caring for opals. Because doublets and triplets consist of multiple layers glued together, prolonged exposure to water will eventually cause lifting between the layers and the infiltration of water. A doublet or triplet will take on a 'foggy' or grey appearance if this happens. This does not mean your opal will be ruined if you wear it in the shower once, or are caught in the rain. It takes prolonged exposure to cause water damage to a doublet or triplet.
Cleaning Your Opals
Solid opal should be cleaned gently with mild detergent in warm water and a soft toothbrush or cloth. Avoid bleach, chemicals and cleaners. Doublets & triplets may be wiped with a damp soft cloth and mild detergent, but should never be soaked or immersed. Never allow anyone to clean your opal in an ultrasonic cleaner, as the intense vibrations may cause cracking in a solid opal, and water penetration in a doublet or triplet. If your stone loses its shine or becomes scratched, bring it back to an opal cutter. After years of wear, small scratches and scuff marks cause an opal to lose its shiny polish and become dull looking. Professional polishing can bring new life to an opal which has become dull or scratched, and we can also check for claw damage and ensure the security of the setting.
Storing Your Opals
If you need to store your opal away for a period of time, simply place it in a padded cloth bag for protection and store it away. For longer storage periods, place your opal in a sealed plastic bag wrapped in cotton wool with a few drops of water just to be safe. The water is not intended to soak into the stone (as opal is impervious) but will prevent water coming out of the stone if it is exposed to very low humidity environments (for example, zero humidity storage safes).