Boulder Opals

Monday, 25 July 2022
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Boulder Opals

What is Boulder Opal?

Boulder opal is renowned for being spectacularly colourful. They are mainly found in Australia, primarily Queensland, and although they do exist in New South Wales, Canada and Brazil. Boulder opals are highly regarded and actively sort after with the most popular colours being green and blue, although the most desirable are red in colour and considered quite rare and therefore valuable. The colours may change if the viewing angle is varied and this adds to the appeal.

What makes Boulder Opal different is that the opal seam is contained within the surrounding rock or attached to it. The decision how to present the opal in the beat aspect is left to the skill of the cutter who has two main options. Either display the seams and patches of the opal as it appears in the host rock. Or cut so that a thin seam of opal is seen as the face and the host rock forms a backing to it.

How Do Boulder Opals Form?

Boulder opals are generally found in cracks, crevices and cavities.

There are a number of hypotheses as to the formation of boulder opals. One theory is that silica originating from dissolved sand which trickles down into cracks and crevices is precipitated out of the solution and forms the boulder opal.

Another explanation is the presence of pyrite (Iron Pyrite often known as Fool’s Gold) which when dissolved by rainwater produces Sulphuric Acid which dissolves the silica in the stone which flows into the cracks and crevices and produces the boulder opal.

How are Boulder Opals Mined?

The majority of boulder opals are mined in Queensland by individual propspectors or small claims using the open cut process. This consist of removing the top layer or overburden and exposing the opal layer. The boulders containing the opals are then cracked open to reveal if they contain any of the gemstones. Within any deposit there can be several types of opal and lots of rock needs to be cracked open to reveal any opals contained therein. Each mine will produce opals with different characteristics.

How Opals are processed

Once the opal bearing rock has been isolated it is cut into slabs using a diamond tipped saw blade. It is then trimmed to expose the opal section and ground down to expose the opal face. The opal face is then polished with a series of grits until a polished surface appears. Final polishing is done with a tin oxide paste and a leather pad. Most of this work is done at the mine so the miners value add to the gemstone to maximise the return on their investment.

Miners tend to work in mine camps which are temporary and subject to a mining lease, usually for a period of ten years. Leases are granted on the understanding that the site is returned to a pre-mining state and the land and vegetation reinstated.

 What is so Special about Boulder Opal?

Boulder opals are primarily found in the areas around Yowah,  Quilpie and Winton in outback Queensland. They are characterised by vibrant displays and bright colours. The iron basis of boulder opals gives stability and depth of colour along with unusual and interesting natural shapes which produces interesting and unusual jewellery.

How to care for Boulder Opals

Boulder opals are easy to take care of and do not need any special care and attention, being stable and safe in water. There is a degree of brittleness and care needs to taken that the gemstones are not dropped on to a hard surface as it may chip. In essence exercise caution when doing manual work and remove before gardening, cleaning or general manual tasks

What is the value of a Boulder Opal?

It is difficult to ascertain the value of boulder opals, but a general rule is the more colours in the opal the more valuable and therefore expensive the opal. Red Boulder Opals tend to be the most valuable followed by Orange, Green and Blue with a black specimen more valuable than a lighter coloured one.

Author Bio

Jill Hansen runs Anastasia’s Of Broome and is a highly recognised expert in both the fields of South Sea Pearls and Diamonds. Jill trained in seeding pearls in the Cook Islands by Japanese Pearl Technicians, and established a wholesale pearl business selling pearls all over the world. She pioneered the combination of pearls and diamonds to create Lust™ Pearls, a unique fusion of these two beautiful gemstones. Jill holds the prestigious recognition of being an Antwerp Diamond Broker for selecting and sourcing Diamonds direct from the diamond capital of the world, Antwerp. She is a diamond expert and is happy to use and impart her knowledge to help customers make informed decisions.


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