Opal is one of the most popular stones in the world today. Its colour variations can be spectacular with bright reds, blues, greens, purples and everything in between. But what is opal and how does it occur? What process creates these brilliant colours that we see?
One of the ways science defines a mineral is that which has a crystalline structure; Opal does not have such a structure, however it is generally referred to as a mineral. Despite this, Opal is in fact a 'Mineraloid' which is essentially a term that describes a geological substance which is lacking in a crystalline structure. Most mineraloids are still referred to as 'minerals' even though this is not strictly the case.
Opals colour was a mystery for quite some time, with many theories being put forward in the early days. It wasn't until the 1960's, with the invention of the electron microscope, that it was discovered that opal was made of up of billions of tiny silica spheres all arranged in an orderly fashion. These spheres are roughly between, in the case of precious opal, 150 to 400 nanometers in diameter. This collection of silica spheres is formed via water mixed with silicon dioxide making its way through cracks and voids in the earths surface. Most of the water then evaporates (opal does have a small water content) leaving behind the tiny silica spheres. It is said that it takes some 4 million years for 1 centimetre of opal to form at a 45 metre depth.
The tiny spheres that make up the stone then diffract the light that enters the stone and divides it into the spectral colours. The colour shown is dependant on the size and arrangement of the tiny silica spheres present within the stone, and the colour patterns changes with the angle of the piece. If the spheres become too compact or unaligned, then the light will run straight through, without diffracting, creating opal with no colour play (this is commonly known as 'potch' or 'common opal').
Pure opal is generally colorless, with impurities giving each piece its background colour. black opal, and opal with an orangey look to it have Iron, manganese oxides and organic carbon impurities giving them their background colour, whereas 'milky' or 'jelly' opal (such as that common in Coober Pedy) has tiny air pockets within the stone.
It is interesting to note that 97% of the worlds opal is mined in Australia with 80% of that coming from the mines of South Australia including Coober Pedy, Andamooka and Mintabie. Other opal deposits around the world can be found in the US (Particularly Nevada), Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil , Honduras, Guatemala, Nicaragua and Ethiopia. In an amazing announcement back in 2008, NASA also indicated that there were opal deposits found on Mars!
From a personal point of view, opal is one of my favourite stones to cut, although certainly comes with its own difficulties which can be somewhat frustrating. The colour play can quite often only be found within thin bands running through the material. Given the soft nature of the stone, this can quite often result in cutters grinding away too much material looking for better colour and ending up with no colour at all! This alone has proved enough to make a lot of people shy away from cutting a good bit of opal. My suggestion for those just starting out is to try your hand with some potch first, before going for the good stuff. At least this can give the cutter some sort of idea of how the stone runs across your grinding wheels and can give you a 'feel' for the material. The end result of a well cut opal can be stunning and makes the frustrating nature of the stone very much worthwhile!
I have put up photos of a few of my opals on this post, but you can see more of my work with this amazing material by checking out my store
here. Until next time!
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