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Note before starting: While phrases like 'real,' 'fake,' and 'genuine' are commonly used in everyday language, they can be problematic and misleading when discussing gemstones. Instead, using 'natural' and 'artificial' is a better way to label materials. Read more about this here.

Opals are renowned for their mesmerising play-of-colour, making them one of the most sought-after gemstones in the world. While natural opals are treasured for their unique beauty, the market also offers synthetic opals.

What are Opals?

Opals are formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone. This silica-rich solution finds its way into cracks and voids. When the water evaporates, it leaves a silica deposit, forming opal. Natural opals are known for their iridescent colour play, which changes with the angle of view and light.

Synthetic opals, created in a lab, replicate the physical and chemical properties of natural opals. They are often made to offer a more affordable alternative to natural opals and to provide a sustainable option that doesn't require mining.

Synthetic Vs. Natural Opal

The most notable difference is in the pattern and colour uniformity. Price can also be an indicator but is not a reliable source.

Gilson Opals

A Gilson opal is the most common and convincing synthetic on the market. They were developed in the 1970s by Pierre Gilson, and they take about a year to produce. There are also Gilson-like opals, which are resin-free synthetic opals made using the Gilson method.

One way to test for a Gilson opal is to view it from the side. A Gilson opal will show a column-like structure not seen in natural opals.

Column-like structures found in Gilson opals


The best way to test opals is with a 10x loupe, as Gilson opals have a diagnostic "lizard skin" or "chick wire" appearance.

 

"Lizard skin" or "chicken wire" appearance.

 

Synthetic opals typically display very regular patterns and colours, while natural opals have a more random, varied appearance.

  

Left: natural precious white opal, centre: natural black opal, right: synthetic Bello opal



Slocum Stone

A Slocum Stone is a synthetic opal developed and patented by John Slocum in 1974. It's an artificial gemstone designed to imitate the appearance of natural opal. It is not commonly found today, but you may find these more in antique jewellery. It is a silicate glass with opalescence produced by light interacting with thin translucent flakes of an iridescent film.

 

 

Opal Doublets and Triplets

Opal doublets and triplets are types of assembled opal gemstones designed to provide a more affordable alternative to solid opals. These are artificial opals and should be labelled as composite materials.

They usually consist of a slice of natural or synthetic opal with a black backing (doublet) or sandwiched between a black backing and a colourless dome (triplet).

Identifying these is easy as you can see the layers; however, if these are set in jewellery where you cannot view them from the side or the back, it will be trickier to identify.

  


Left: opal doublet showing a slice of natural opal on black back, centre: opal triplet with a black base, clear dome and a thin slice of natural opal in the centre, right: thin slice of opal prepared for a triplet. Photos: The Australian Opal Shop

 

Summary

There are many different types of opals on the market, many which I have not touched on here and there are many convincing synthetics. Knowing what to look for can help you make a more informed purchase. As always, it's best to purchase from a reputable source and consult an expert if you are unsure.

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