Behind the Opal Essence

A guide to Opal quality




Boulder Opal was discovered in Australia at Quilpie, in Western Queensland, in about 1870. Since then it has risen to recognition as the second most valuable type of opal, however remains largely unknown to the general international public. Boulder opal is generally mined in open cut operations, where large amounts of dirt are cut away from the surface and removed. The ironstone boulders which contain the opal are referred to as 'floating boulders' due to their irregular positioning under the surface. The opal usually forms as thin veins within these boulders, and most stones are cut with the host ironstone still remaining on the back.


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.



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

Black Opal - Black opal may display all the brilliant colours of the rainbow, in all the patterns which opal displays, such as floral, ribbon, harlequin, straw, chinese writing, rolling flash, etc.

Boulder Opal - Boulder opal is identical to black opal in its spectrum of colours, its potential for brilliance, and in the patterns which it may display. Some argue that because boulder opal is a thin layer of opal located very close to a dark backing, it has the potential for brighter & better colour, however this is debatable.

Note: Gem quality stones (gem quality refers to extremely high quality opals) - In proportion to the amount of opal mined, there is very little difference in the quantity of rare, high quality stones which are found of each type of opal.

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.


Boulder Opal - Due to the thin nature of the opal veins which form in boulder opals, it is impossible to cut domed cabochons in most boulder opal stones. The stones are therefore usually cut into free form, irregular shapes (given the odd exception) to maximise the size of the stone and minimise the loss of opal. Boulder opals therefore often cater to a slightly different market and appeal to those who like irregular shapes in 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.


So there you have it. Hopefully now you'll have an understanding of the difference between black opal and boulder opal, what makes them different, and why sometimes their value differs. It's all part of the education process!


So which is better? The truth is - neither is better, they are both equal in quality. It all depends on the individual stone, your individual tastes, and what you value in a stone. If you value the rarity of the type of stone (the satisfaction of owning something rare), the high profile often associated with it (e.g. pink diamonds or a Rolls Royce), or an oval, dome shaped opal (as opposed to a free shape stone), then you would probably prefer a black opal over a boulder opal. More often than not, it won't matter what type of opal it is, once you see the one for you, you'll fall in love with it!