Opals were formed after the sea that previously covered 60 per cent of Australia started drying out – from Coober Pedy in South Australia, (through Lightning Ridge in northern NSW) and all the way up to the Gulf of Carpentaria in northern Australia – this was caused during an “extraordinary episode of acidic weathering”.

This all occurred between 97 and 100 million years ago. As a result, pyrite minerals, commonly referred to as fool’s gold began releasing sulphuric acid. This drying out of Australia’s centre increased the acidity levels at a shallow depth, releasing silica through the weathering of sandstone. Further weathering and climate change then lowered the acidity to a level at which precious opal could form into a silica-rich gel. This silica gel was picked up by water and the deposition worked its way down through cracks of the sandstone within the earth at an estimated rate of 1 centimetre every 5 million years.

Central Australia is believed to be the only place on earth where acidic weathering of that scale took place.

Black Opal from the Lightning Ridge fields were discovered in the 1880’s. Black Opals are the most valuable Opals on earth, simply due to how rare they are. The dark body tone (shown as N1 – N4 on the “Scale of Body Tone” below) result in the most beautiful ‘play of color’ throughout and on top of the opal gemstone.

Very high Gem quality Black Opals can fetch prices of up to AUD $20,000 per carat. (But remember, just because an opal is black, doesn’t necessarily mean it is valuable).

There are many factors in pricing Opals, these include (but aren’t limited to); body tone, brightness, shape, carat weight and pattern. These characteristics all make part of determine the overall value of an Opal.

Read more about valuing Opal here Pricing Opal.

Dark Base Opal has a smokey to dark grey body tone, labelled as N5 – N6 on the “Scale of Body Tone” (shown below), and can be found at Lightning Ridge NSW.

In South Australia, Mintabie is famous for Semi-Black and Dark Crystal Opal, as are certain fields at Andamooka and Coober Pedy.

In the Australian state of South Australia, the Opal fields account for, and produce an amazing 80% of the world’s supply of White Opal.

White / Light Opals have a body tone ranging from a milky white to transparent. (shown as N7 – N9 on the “Scale of Body Tone” below).

White / Light Opal is the most common variety of Opal found at most fields, but, these opals are mainly found at Coober Pedy. Which is a town in the state of South Australia, famous for its White Opal.

White / Light Opal can display the full colour spectrum on an opaque background, whereas Crystal Opal is transparent to translucent without the milkiness within the stone, and has bright colour flashes suspended in its midst.

Crystal Opal is superior to White Opal, Grey Opal and Jelly Opal. Jelly Opal exhibits a moderate play of colour within a transparent background.

The Australian state of Queensland is the home to the ‘Earthy’ body toned Boulder Opal.

Opal from the boulder fields which now span over 1,000km, were discovered in about 1870.

Boulder Opal can be light or dark in appearance and can also form as veins within the ironstone  host rock.

Boulder Opal may display the same darkness and brilliance as gem Black Opal from Lightning Ridge, though, Boulder Opal is very a durable gemstone due to the ironstone backing in which the opal forms on, which the cutter retains and polishes during the lapidary process.

The water content in Boulder Opal is very low, it hardly ever cracks or crazes as it ages and is considered the most stable type of natural Opal on earth, by many experts.

There is very little difference within the amount of very high quality gem opal mined out of the grounds of Lightning Ridge, vs the Boulder fields of Queensland.

There is much debate on whether Boulder Opal has a better and brighter color over its Black Opal brother, due to its thin layer of opal located very close to its dark ironstone backing.

Black Opal and Boulder Opal both display all the brilliant colours of the rainbow, and both display common patterns, for example; floral, ribbon, harlequin, straw, Chinese writing, rolling flash, etc. Yet, Boulder Opal is much more affordable and is generally sold at approximately one third of the Black Opal carat price.

Another difference between Boulder Opal and Black Opal is that when grading Black Opal, and identifying it, we refer to the “Scale of Body Tone” chart (pictured below) to decide whether the Opal is Light, Dark or Black.

A difference between White/ Light Opal, Black Opal and Boulder Opal is that traditionally, White and Black Opals are generally cut symmetrically, in oval or round shape, with a domed top (also known in the industry as a cabochon), as these are the most popular shapes in jewelry manufacturing globally.

(I should now point out that the demand for these traditional (symmetrically) cut shapes are changing. We are seeing many more free-formed (baroque) shaped Opal Gemstones entering the market due to advances in jewellery making).

Due to the thin and undulating nature of the veins which form Boulder Opal, it is near impossible to cut domed cabochon stones. Boulder is therefore usually cut into free-form irregular shapes to maximize carat weigh of the stone (to minimize the loss of opal). Boulder Opal also cater to a slightly different market of people who like irregular shapes in jewellery, and dark earthy body tones.

Black Opal (along with White and Boulder Opal) are classed as having a hardness of 5.5 to 6.5 on Moh’s scale of hardness. (Tanzanite is classed at 6.5 and Diamonds are classed as a 10 on this same scale).

Opals are similar to the hardness of glass. The opal layer within boulder is the same, however the ironstone backing which is naturally attached to the gemstone, gives Boulder Opals an extra hardness and strength which gives it an advantage over other types of opal. Boulder Opals will generally stand up better to impact and be much less likely to crack than a Black Opal.

B1 – Brightness – Faint:

Shows color only when under direct sunlight. Even then, the color is very faint.

B2 – Brightness – Dim:

Shows a tiny bit of color under low light. But under indirect sunlight, or under a grading lamp, the color is very dim.

B2.5 – Brightness – Somewhat Bright:

Shows better color under low light, and also under indirect sunlight. Showing more life than ‘Faint and Dim Opals’.

B3 – Brightness – Bright:

Shows a fair amount of color under low light, and very nice color under indirect sunlight or under a ‘grading lamp’.

B3.5 – Brightness – Bright +1:

Shows very nice color under low light, and good color under indirect sunlight or under a ‘grading lamp’.

B4 – Brightness – Very Bright:

Shows very good color under low light, and crisp color under indirect sunlight or under a ‘grading lamp’.

B4.5 – Brightness – Extremely Bright:

Shows very good color under low light, and exceptional color under indirect sunlight or under a ‘grading lamp’.

B5 – Brightness – Brilliant:

Shows exceptionally bright and crisp mirror-like color under low light, under indirect sunlight or under a ‘grading lamp’. This is often bright in the shade, or even at night.

“Doublets” (as Doublet Opals are referred to within the Opal Industry) consist of two layers of Opal, adhered together with an epoxy, this is generally mixed with black chalk powder, or black oxide to darken the back of the gem crystal opal used in the Doublet.

The backing used in a doublet is generally BLACK. This can be black potch (common opal), black industrial glass, black hard plastic, or dark boulder opal ironstone.

The top of a Doublet, consists of a thinly sliced piece of gem Crystal Opal, or White Opal. The thickness of the opal in a doublet can vary, however it is generally much thicker than the piece of opal found in a Triplet Opal. The top of the stone is generally ‘cabbed’, to give the stone a cabochon (domed) look on top.

Doublets can easily be identified by looking at the side of the opal. Where the stone has been adhered together, you will notice that the line where the colored opal and the black backing meet is perfectly straight.

When a Doublet Opal is set into a piece of Jewelry, with the sides covered, it is near impossible, even for an expert, to tell whether it is a doublet or a solid opal (as the top of the stone consists of pure opal). It appears just like a Black Opal, and this is why Doublet Opals have a much more natural appearance than triplets, and are sought after, at a fraction of the price of a Black Opal.

“Triplets” (as they’re referred to within the Opal Industry) consist of three layers, adhered together with a clear epoxy.

The Backing of the stone, is a Black backing, the same as a Doublet Opal. This is mentioned above, in more detail within the “Doublet Opals” section.

The Center of the Triplet Opal is usually a paper-thin slice of Gem Crystal, or White Opal.

The clear ‘Cap’ on top is usually a piece of glass, or a quartz cap, or a plastic cap. Either way, the capping is usually always in a dome shape. The clear capping also magnifies the color of the opal slightly, and serves to protect the opal slice inside.

It is easy for an experienced opal professional to immediately identify a Triplet Opal, simply by its appearance. Triplet Opals usually have a ‘glassy’ appearance as the light reflects differently from the top of the stone. If you look side on at a Triplet Opal, you will see a straight line where all the layers meet, and you should also be able to see a ‘clear’ / ‘see-through’ effect where the cap of the Triplet sits.

Because there is very little “Real Opal” within a Triplet. They are normally cheaper than doublets. Also, as the top of the stone is generally made from synthetic materials, Triplet Opals can be a lot more resistant to impact than solid or Doublet Opals.