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Behind the Opal Essence

A guide to Opal quality

Looking after your opals

Opals are among the most delicate gemstones that are commonly worn, and require some special care. With a little knowledge and some common sense, you can keep your opal in beautiful condition for many years to come.

There are two main reasons why opals are regarded as delicate. One reason is that they are relatively soft gemstones (between 5.5 and 6.5 on the Mohs hardness scale), so they can be easily scratched, even by common household dust, which often contains quartz. Another reason is that they have significant water content - from 3 to 30 percent. If an opal is kept in an environment with very low humidity, it can dry out and crack. Due to their water content, opals are also sensitive to sudden changes in temperature.

We do not stock opal doublets and triplets but these have an additional vulnerability. Opal doublets and triplets consist of a layer of precious opal glued to a black backing. Since the layers are glued together, prolonged exposure to water can weaken the glue and allow water to penetrate the space between the layers. The opal may then take on a gray or foggy appearance. Solid opals are not vulnerable to this problem. Solid opal should be cleaned gently with mild detergent in warm water using a soft brush or cloth. Bleach, chemicals and cleaning fluid should always be avoided. Doublets and triplets may be wiped with a damp soft cloth and mild detergent, but should never be soaked or immersed.

Opals should never be cleaned in an ultrasonic cleaner. The intense vibrations created may cause cracking in a solid opal, and water penetration in a doublet or triplet.

If you need to store your opal for a long period, it is a good idea to keep it in a sealed plastic bag with a damp piece of cotton to prevent dehydration. The best way to avoid damage to an opal is preventative care. Opals are best suited for earrings, brooches and pendants, since these types of jewellery are less exposed to wear and tear. An opal ring should really only be worn on occasions, not on a daily basis and the opal will still probably need to be re-polished from time to time to maintain its lustre.

The Boulder opal found in Queensland forms in a slightly different method to other types of opal, forming inside an ironstone concretion. The concretion was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2at 28%, Fe2O3 + AL203 at 68% and H2O at 1% composition.

Boulder Opal Composition


Silicon Dioxide


Fe2O3 + Al2O3

Iron Oxide + Aluminium Oxide




Opal Formation

The opal forms in generally elongated or ellipsoidal ironstone concretions or boulders, from a few centimetres, to up to 3 m across. The boulders may be confined to one or more layers or randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides).

The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts. (as found at Yowah and Koroit fields, the famous 'Yowah-nuts'). Example: Aurum’s ‘Viridian’ Butterfly Brooch.

Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone). Rare seam or band opal is also found and is typically encased in ironstone.

Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled. Wood opal is occasionally found replacing woody tissue material.

As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone, and stones are usually cut with the natural ironstone backing intact. Solid opals may be cut from the ironstone material where the opal is of sufficient thickness.

Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage. Example: Aurum’s ‘Chartreuse’, ‘Azure’, and ‘Periwinkle’ Pendants. Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use.

Boulder opal also has a dark body tone (although there are some occurrences of white boulder opal), which is as dark as black opal. Because boulder opal forms in thin veins in ironstone boulders, the host ironstone is usually left on the back of the stone. This is why, if you look at the back of a boulder opal, there is a layer of brown rock attached to the back. For this reason, boulder opal is sometimes referred to as a 'natural doublet', as this layer of dark stone on the back gives the opal its dark body tone.

Comparing Boulder Opal with Black Opal
Colour & Pattern

Jewellery, precious stones and metals possess a mysterious significance, enhancing their own natural fire and warmth, and they achieve that which man himself cannot- that is to escape from time itself.

From the earliest times the scarcity of precious minerals – and the difficulties frequently attendant upon their discovery and divining from the Earth – has given them a value that made them symbols of status and social standing as well as laying the foundations for investment and the roots of the economy we know today.

The sourcing and acquisition of these amazing gemstones is of extreme importance to any prospective item of jewellery. They are intrinsic to Aurum’s success and certainly only the most breathtakingly beautiful high quality stones are, selected personally by Richard and his daughter Julie, from diamond specialists in Antwerp and from visits to stone cutters in the far East.

Stone Shape

Black Opal - The market for black opal generally demands symmetrical, oval shaped stones, with a domed cabochon, as these are the most popular shape in most jewellery.


Black Opal - Black opal is the rarest form of opal, and is only found in opal mining fields approximately within a 70 kilometre radius of the town of Lightning Ridge. Black opal is becoming increasingly rare and top grade black opal is not currently found anywhere else in Australia or in the world. (Unlike diamonds, which are in fact very common, black opals are genuinely rare.)

Boulder Opal - Boulder opal is currently much more readily available than black opal. The area which has the potential to yield boulder opal is much larger, and a relatively small proportion of this has been explored. The area stretches along a 200 to 300 kilometre strata in Western Queensland. Therefore, the future looks promising for boulder opal mining, as it is likely to become the only actively producing source of natural dark opal. (This is partly speculation of course).


Black Opal - Because of its rarity and status, black opal carries with it a certain price attachment. Market forces determine that rare items which are highly sought-after fetch higher prices than more common items. Because of their beauty, rarity, and status, black opals fetch a much higher price in comparison to boulder opals. Some argue that black opals are overpriced, while others state simply that market forces (supply and demand) determine their value.

Boulder Opal - Boulder opals are relatively under-priced in comparison to black opals. A high quality, predominantly red black opal, which is identical to a boulder opal, may fetch prices up to seven times that of the boulder opal. Generally speaking though, boulder opals are considered to carry one third of the price of black opals, despite the fact that the cost of mining boulder opals is much more than that of black opals. Open cut mining requires much more machinery and fuel than shaft mining.

The reasons for this are as follows;

Rarity - as stated before, boulder opals are found more commonly than black opals. Fame & Status - Black opals are renowned throughout the world for their rarity and beauty. The Ironstone Factor - Due to the natural ironstone backing and the thin nature of the opal layer, boulder opal is not traditionally priced 'per carat'. Because ironstone is much heavier than opal, valuers consider that inclusion of the ironstone would be a distortion to a 'per carat' price, and therefore a lower value is applied to the stone overall. Boulder opal therefore has different valuing standards applied to it, and is sometimes valued 'per piece' rather than at a price per carat. It's important to note, however, that black opals which have a colourless black potch backing are still valued per carat, and do not receive any kind of 'penalty'. Practically speaking, it makes little difference what is on the back of a stone once it is set into jewellery - colour and brightness are still the most important factors.

Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled. Wood opal is occasionally found replacing woody tissue material.