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!
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