An Introduction To Opal

Opal, most magnificent of precious gems…

No two opals are alike.

Opal distinguishes itself from other gemstones because of its phenomenal Play of Colour, incredible range of patterns and vibrant colours, which change and flash as you turn the gem in your hand.

Opal is a non-crystalline form of hydrated silica dioxide – chemically, it’s similar to a whole range of silica-based minerals – but what makes opal special is its microscopic structure.

At a microscopic level opal is made of tiny spheres.

In precious opal, the spheres are arranged in tightly packed, ordered three-dimensional arrays. When white light hits the spheres and the spaces between them, it is diffracted back as visible colour.

The size of the spheres and how loosely or densely they are packed determines what colours are absorbed and what colours are able to be transmitted.

The groups of neatly ordered spheres usually occur in patches and it is these patches, and the interactions between them, that produce the pattern and play of colour in opal. This video shows how the play of colour changes in this piece of opal as the light source is moved and therefore the colour diffraction in the different colour patches changes.

This video also demonstrates what an exciting natural gemstone opal is.

Watch the colour flash on this opal!

This is a piece of rough opal – revealed for the first time in millions of years!

This is the way it comes out of the ground, amazing! There’s no other gemstone like it!

Only the tiniest fraction of the opal found exhibits play of colour, because it is so rare for the right conditions to occur to form the right tightly packed arrays of spheres.

In the vast bulk of opal, the silica spheres are too large or too small to diffract visible light, or the size of the spheres varies too much for them to be able to pack tightly together and form a diffraction grating.

Black opal nobby split

This kind of opal is called common opal – in Australian opal it’s colloquially known as potch. In this photo you can see black potch around the green-coloured precious opal within the claystone that our opal forms in at Lightning Ridge.

Potch is actually very important, especially in the case of black opal. This wonderful rough specimen is in our private collection, it was broken open during the mining process. You can see the black potch beneath the blue green gem opal.

This piece of spectacular seam rough opal also clearly shows a bar of gem colour sitting on a layer of black potch.

It’s this dark potch that makes black opal black: it’s when the opal naturally forms with gem colour directly adjacent to black or dark potch, and when the cutter can orient the stone so the gem colour sits on top of the potch. The dark body tone of black opal gives it the most rich, luscious colours imaginable.

Opal of many kinds – black, grey, white, crystal – can be found in seam formations. Seam is more abundant than nobby opal and is found in sedimentary deposits at Lightning Ridge, the Grawin-Glengarry-Sheepyard area, White Cliffs, Coober Pedy and the other South Australian opal fields. When seams are broken open during mining, opal colour, if it is present, is often revealed in cross section.

This is the typical miner’s heartbreaker – bright gem opal, but so shot through with sand that it cannot be cut into a commercial cabochon. Precious opal is more rare than ruby or emerald – and far, far, far more rare than diamond. It’s a tragedy to throw it away or grind it to dust on the cutting wheel.

Sand shot opal nobby

Carved Semi-faceted Crystal

Magnificent gem green and blue on black.

Carved black opal

So in some cases we can... by painstakingly removing the sand and other flaws and carving then polishing the opal into a beautiful, absolutely unique shape that conserves the precious opal. It gives us amazing opal carvings like these. Note the carved semi-faceted crystal; 'Crystal' opal is named for its translucent quality even though it does not have a crystalline structure. Note also that although Lightning Ridge is most famous for its black opal, it also produces fine crystal, black crystal, grey and white opal.

These little nobbies are called pea nobbies and they can produce very beautiful little gems indeed.

Nobbies sometimes occur in pockets and sometimes in isolation. The vast majority of nobbies are potch without any gem colour, or the colour distributed in such a way that a gemstone cannot be cut from the piece.

Digging for nobby opal is like looking for the coin in a gigantic underground plum pudding made of sandstone and clay. And the coin mightn’t even be in your pudding – it might be in the pudding next door. And even if you’re lucky enough to find a coin, there’s a very high chance it’ll be very ordinary looking, instead of shiny and beautiful.

But oh boy, when you find it, there's nothing like it on Earth!

Opal is also found in horizontal seams of varying thicknesses, and seam opal can produce incredible patterns like this one.

Gem nobby cut stone

This is amber-coloured common opal known locally as honey potch, amber potch or sometimes just 'amber'. If you're lucky you might find some with 'sunflash' colour.

Large Opal Nobbies

So how does this all happen? We know about the microstructure that produces play of colour in precious opal, but how does it form?

That’s actually still quite a controversial subject for geologists and gemologists. Scientists are still debating whether or not heat, pressure or even bacteria are involved in opal formation. The predominant theory is called deep weathering, where silica on the surface of the earth became soluble through acidic oxidative weathering, worked itself downwards via pathways through mud and sandstone, and deposited into non porous voids and seams in the dense clay immediately underneath the sandstone roof. Over time the clay absorbed the water from the silica gel and left behind the solid silica as a nobby or seam.

And if the cavity formed because a bone or shell or plant fragment was once buried in the sediment which later fossilized then the phenomena called an opalised fossil occurs.

Opalised Mussel shells approx. 110 Million years old, from our mine!

Opalised Mussel Shell displaying multi gem bars of colour!

Cross-section of an Opalised Mussel in our private collection

Opalised fossils are extremely rare and prized throughout the world

Beautiful Coober Pedy light opal pair displaying broad Floral Flash pattern

Opal is actually found in several places across the globe, including the United States, Canada, Mexico, Brazil, Indonesia, Ethiopia, Honduras and Hungary – though the Hungarian opal is well and truly mined out.

Lightning Ridge is best known for its Black and Crystal Opal.

White Cliffs, west of Lightning Ridge in NSW, is famous for its white opal and although not much opal is coming out of White Cliffs now, it was at one time the world’s largest producer of opal. South Australian towns such as Coober Pedy, Andamooka and Mintabie are best known as producers of light and crystal opal.

And in western Queensland there a number of locations that produce boulder opal, in which the gem opal forms in cracks and cavities within brown ironstone boulders. Boulder opal can be very dramatic and beautiful.

Boulder Opal displaying striking broadflash pattern

Exquisite rare multi-colour Boulder Opal