Australian Opal Guide


 

Types of Opal
History & Formation
Valuing Opal
Caring for Opals

Types of Opal

Black and Semi Black Opal

Black Opal

Black Opal is solid opal distinguished by the black or very dark body colour of the stone and is highly sought after because of its beauty and rarity. Semi Black opal has a background which ranges from smoky grey to nearly black.

The dark backgound accentuates the play of colour in the opal and the brighter and sharper the colours, the more valuable the opal.

Most Black Opal comes from Lightning Ridge in New South Wales or Mintabie with small amount coming from the fields around Coober Pedy. See our Black Opal page for more detail.

White Opal

White or light opal is solid opal distinguished by a white or light background which is opaque. The play of colour is generally more subdued than black opal however, quality white opal can be brilliant and display many colours.

White Opal generally comes from Coober Pedy and Mintabie in South Australia. See our White Opal Page for more detail.

White Opal

Crystal Opal

Crystal Opal

Transparent or translucent opal is defined as crystal and here the colors are sharp and visible below the surface. Crystal opals come in all color variations and display bright colors. Crystal opals can be either light or dark and are solid.

Crystal opal is clear enough to see through against a light surface but when viewed on a dark surface the colours really spring to life. In terms of value, the more transparent the stone is, the more it is worth. See our Crystal Opal page for more detail.

Fire Opal

The term "fire opal" is commonly used to describe the clear orange crystal opals which come from Mexico, some of which have a play of colour. Many have just an orange or reddish base with no play of colour and most of these stones are faceted.

The term "fire" is also used to describe the dominant colour reflected or diffracted out of any opal.

Fire Opal

Yowah Nuts

Yowah Nut Opal

Mined at Yowah in western Queensland Yowah nuts are opal deposits which have formed inside a nut like kernal of ironstone.

They are usually very hard and can be polished to a very high gloss and are often made into attractive jewelry.

Matrix Opal

Andamooka matrix is a very porous opal which when mined is light in colour. It is cut and polished then carbonised by treating it with sugar and sulphuric acid. The end result is a stone which looks very much like good black opal. It has a characteristic salt and pepper look and can be easily identified when looked at under a magnifying glass.

 

Matrix opal can also be formed naturally where the silica has run into the minute cracks and crazing in ironstone boulder and this form is known as boulder matrix

Andamooka Matrix Opal

Boulder Matrix Opal

Boulder Opal

Boulder Opal

Boulder opal is a naturally formed solid opal which consists of a fine layer of opal that has been deposited by nature on and in the fissures of ironstone base rock. Because it geneally has a black or very dark background colour it has the appearance of black opal.

There are often inclusions in the opal layer and because the layer follows the contours of natural ironstone the shape is often very undulated. Depending on how the stone is polished it can either be very smooth or be "pockmarked" where ironstone protrudes to the surface. The irregular shape of boulder opal makes it a designers delight and most pieces of jewelry using boulder opal are very unusual.

Boulder opal is found at a number of locations in Queensland with some small quantities coming from Andamooka in South Australia.

Doublet Opal

Doublet Opals consist of a layer or slice of natural opal which has been bonded to a dark or black backing. The backing can be potch, black glass or ironstone boulder. These days most doublets are bonded to an ironstone backing thus forming a "boulder doublet". The dark backing enhances the colours and has the effect of simulating high quality black opal. However because less opal material is required doublets are much cheaper.

Opal Doublet

Triplet Opal

Opal Triplet

Triplet Opals are similar and consist of a thin slice of natural opal bonded to a black backing together with a glass or crystal dome on top which protects the opal. Triplets range from souvenir quality to gem quality with the best specimens showing the brilliant colours of fine black opal. However because only a thin slice of opal is used the cost is much lower.

Doublets and triplets are assembled by man and do contain natural opal - they are not synthetic or imitation opals which do not have the same chemical composition as natural opal.

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History and Formation

Opal has existed for many millions of years and it is thought that the the name opal is derived from the Greek "Opallus" which means to see a change in colour.

Precious opal is very rare and is found in only a few locations in the world because it requires very special conditions in order to form. We are very fortunate that this did happen on a such large scale in Australia which now produces over 95% of the world's precious opal.

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Approximately 140 million years ago much of central Australia was an inland sea and fine mineral sands rich in Silica were washed up on its shores. The sea eventually receded and formed what is now kown as the Great Artesian Basin. Around 30-40 million years ago, heavy weathering disolved some of this silica and formed a gel solution. This gel then seeped into crevices and cracks in the ground and over many more millions of years eventually hardened, forming what we call opal.

Most of the opal formed was common opal or "potch" which is opal that does not have a play of colour. A very small percentage formed as precious opal which displays the "play of colour" or the prismatic colours of the rainbow that we all know and admire. The difference is not caused by impurities but by the arrangement of the spheres of silica which make up the opal.

Precious opal consists of uniform spheres of silica which form a 3D grid. As white light hits the silica in solution between these spheres it is diffracted and reflected back as different colours, hence the "play of colours". The size and spacing of the spheres determines the dominant colour of the stone. The brilliance of the stone is determined by the uniformity of the spheres and their spacing.

Common opal on the other hand, is made up of silica particles or spheres that have random sizes and spacing which disperse the incident light and therefore have no play of colour.

If we get technical, opal is the gel form of hydrous silicon dioxide, with a chemical formula of SiO2nH2O. It has a hardness of 5.5 to 6.5 which is similar to garnet and has a specific gravity of 1.95 to 2.2. It is a member of the quartz family and has the same chemical composition, however quartz is the crystalline form.

Opal has been adopted as the national gemstone of Australia and is the birthstone for May and October.

 

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Valuing Opal

 

Valuing Opal
Opals are unique and no two are exactly alike. This makes valuation a complex process, however some of the important factors in choosing and valuing an opal are:

Type
There are many types - common are solid, boulder, matrix, crystal etc. Solid opal, particularly black is the most rare and valuable however other factors do heavily influence value.

Brilliance or Brightness
This is the brightness of "fire" coming from the opal and does not refer to the base colour of the opal. In order red is the most valuable with orange, yellow, green, blue and violet following.

Pattern
Good patterns of colours combined with brilliance have a huge effect on value. Broad patterns are more desirable than small or pinfire patterns and distinct patterns such as harlequin, straw, ribbon and rolling flash are very rare.

Base Colour
This is the background colour of the stone. When rating black opal - the blacker the colour, the more valuable the stone.

Shape and Cut
This includes the overall shape and quality of cutting. Generally domed stones are more valuable than flat and regular shapes are more valuable than freeforms. Freeforms however can make very unusual and unique pieces of jewelry because of the random shape.

Size
Opals like diamonds are measured by carat weight. Quality opal can command a price per carat similar to and sometimes more than diamonds.

Imperfections
A clear opal with rich vibrant colours will be highly valued. An opal with inclusions, cracks or opaque patches will not be as valuable as a clear stone. The exception to this is boulder opal where sometimes the extraordinary patterns caused by the ironstone makes it even more rare.

Personal Preference
Unless you intend trading in opal there is no point in getting too carried away with all the technical details. Valuing opal takes into acount all of the above and more and it is rare that even two experts will completely agree.

If you see a stone or a piece of jewelry that is attractive and appeals to you then that is fine. The opal is a stone of rare and unique beauty - each person will see and appreciate it differently.

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Caring for Opals

Opal is a unique and beautiful gemstone which will last indefinitely if properly cared for. Generally speaking you should treat your precious opal with the same care and respect as you would with any fine jewellery.

While there are some special requirements, caring for opal is really very simple providing that you keep in mind two basic facts about opal.

Firstly, most precious opal contains about 6 - 10% water. Therefore opal can craze or crack when subjected to harsh, dry conditions and rapid changes of temperature.

Secondly, opal has a hardness of about 5.5 to 6.5 measured on the Mohs' scale of hardness. Compared to this diamonds are harder at about 10, garnets, the same at about 6.5 and gold is much softer at about 2.5 - 3. Therefore, opal like any other gemstone can be broken, chipped, scratched, or lose its shine with heavy wear and tear.


FAQ

Can I put opals in water?

Yes, solid opal can be wet or soaked in water without any problem. This is not advised if the stone is a doublet or triplet as the water may affect the cement or adhesive bonding the opal layer to the crystal cap or backing layer.

How do I clean opal?

Solid opal can be cleaned with a soft detergent in warm water using a soft cloth or brush. Once cleaned the opal should be rinsed in clean water to remove any residue. Doublets and triplets can be wiped with a damp soft cloth but should not be soaked.

What about oils?

It is not recommended to store opal in oil or glycerin - though oil will generally not soak into the stone. Oily face and hand creams should not cause a problem except that they may build up in or on the jewellery and look unsightly. This is easily cleaned off with a soft brush and warm water.


Can I wear my opal when washing the dishes and gardening?

Prolonged exposure to harsh detergents and other chemicals can damage or "dry out" the stone. Be particularly careful about exposing opals and sterling silver to chlorine or chlorine bleach which will affect it.

Sand and soil are abrasive and ultimately will scratch the surface of the finely polished stone and any gold or silver with it. An accidental hit or knock could crack the stone or damage the metal claws holding it into the setting.

Treat all jewellery with respect and take it off before engaging in any of these activities.

What do I do if my stone loses its shine or becomes scratched?

You should take your opal to a reputable jeweler who knows about opal. If the stone needs re-polishing this can generally be done for a very reasonable price. He can also check for claw damage and make sure that the setting is still sound.

What about doublets and triplets?

Both are layered opals which do need a little more care than a solid stone. Doublets consist of a slice of precious opal which has been cemented to base of common opal potch or more commonly ironstone. Triplets are similar except that they also have a clear crystal cap cemented to the top of the precious opal slice in order to protect and enhance it.

While both can be cleaned with lukewarm water and mild detergent it is not a good idea to wash or immerse them in water as this may affect the cement bonding them together. For the same reason it is not recommended to use an ultrasonic cleaner for cleaning jewellery containing doublets or triplets.

How do I store my opals?

Generally storing your opals wrapped in a soft cloth is all that you will ever need to do.

If you keep you opals in a de-humidified atmosphere such as a safety deposit box or bank vault for long periods some people advise putting them in a sealed plastic bag with a little water or damp sponge to protect them from sudden temperature and humidity changes, however this is not really necessary.

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