Flood Gem Flood Gem – Queensland Boulder Opal
  • Australian Opal Types

    White opal is the least expensive of all the natural opal types found in Australia. The reason for this is because it is the most common, being found around Coober Pedy, Andamooka, Mintabie South Australia, White Cliffs in New South Wales and throughout the vast expanse of the Queensland “opal belt”.

    Coober Pedy and White Cliffs have been the major producers.

    Boulder opal is found in Queensland from Kynuna in the northwest to Hungerford in the south. The deposits are scattered and remote with some mines being almost 100km away from the next.

    Boulder opal consists of a layer of opal naturally adhering to an  ironstone, mudstone or sandstone backing. The opal is found as thin to thick veins running through the ironstone concretions. These are found generally in sandstone or clay levels within the opal country.

    Boulder opal is cut leaving some of the natural ironstone backing under the opal.  Ironstone intrusions through the surface are not uncommon and may or may not detract from the value of the opal.

    A “clean faced” boulder opal with little or no ironstone or sand showing in the face of the gem is the most sought after form of boulder opal.

    The cost of boulder opal is generally much less than a similar looking solid black opal from Lightning Ridge. It is not uncommon for a boulder opal to exhibit much brighter colours than most black opal.

    Matrix Opal; Opal matrix is a form of boulder opal and has thin veinlets of colour running through the ironstone. The range in variety of the different ironstone patterns and colours are staggering.


    Carbonized Sandstone Matrix; (FAIRY OPAL), This is sandstone found in association with pipe opal where the sandstone has been impregnated naturaly with opal. The colour is almost impossible to see in  the raw material. Select pieces of sandstone are soaked in oil then heated to carbonize the oil (turning the piece black) this is then further sawn along colour bars, preshaped and sanded. Better quality pieces are selected  to be taken to the next stage (stabilization). Stabilization is achieved by fully impregnating the sandstone with a stable,inert, nontoxic, polymer resin under controled conditions. (This process is pattented and done professionally outside our business by contract). The resulting pieces are returned to us for finnishing. The final pieces are relatively large piece of lower cost waerable pieces of “Fair Opal” Jewellery.

    Black Opal; This type of opal has been found in all three states with the black opal nobby from Lightning Ridge in New South Wales being the rarest and most sought after gem of all the opal types.

    A true black opal exhibits a play of colour on top of and as part of a natural **potch base. They can be found with only a thin layer of opal on top to the whole stone consisting of gem opal from top to bottom with a very dark body colour.

    This is the most expensive opal of all with prices in excess of $20,000 per carat for gem “red on black” being paid on the field.

    *Crystal Opal: In the opal trade an opal with a transparent body colour is termed “crystal”. Opal is an amorphous substance and has no real “crystal” structure. The term is only in reference to the clarity of this type of opal. Top quality crystal opal is second only to black opal.

    **Potch; This is of the same composition as that of opal however it dose not exhibit the play of colour.

    Doublets; These are made when opal is found that is too thin for a solid stone. The backs of these pieces are flattened and glued to a backing of potch or ironstone. The glue is generally coloured black or the back of the opal is blackened before gluing as to enhance the colour. When made correctly doublets can resemble a high quality black opal and cost about 10% of the price of a similar solid black opal.  However immersing the gem in hot water for extended periods of time may cause the glue to give way.

    Triplets; These are made in a similar fashion. The opal is purposefully cut as an extremely thin slice and the addition of a crystal or glass cap is glued on top to protect the very thin layer of opal. These seem to be even more susceptible to glue failure when immersed in hot water. Triplets are the least expensive of all manufactured natural opal products and are mostly used in souvenir jewellery. Top quality opal triplets with unique patterns can command a high price.

    Imitation Opal; In recent years an effective opal imitation has been produced using microscopic polymer beads fused together to imitate the natural structure of opal. It is important to keep in mind that these are plastic products and will scratch easily. Heating a pin with a lighter and then placing the point momentarily on the back of the piece is a positive test. If a small indentation is left there is no doubt that it is plastic.

    Gilson Synthetic Opal; This is a laboratory produced product that has the same chemical composition as natural opal and exhibits most of the same characteristics as natural opal. It is sometimes difficult to detect to the untrained eye.