How do black opals form

Home » Gemstones » Opal » Black Opal. Black Opal A solid black opal cabochon with a strong blue face-up color play. The term is also used for opal that has a dark blue or dark green bodycolor.

The dark bodycolor often makes the fire of black opal more obvious. This contrast of fire color to bodycolor makes black opals very desirable and sell for high prices. These tiny spheres also determine colours and patterns of opals.

The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, larger spheres produce the reds. And more and more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.

Back to Top. Advanced 1 In this chapter we are going to tell you a more details about the opal. How opal is formed? The secret of colours 3. The colour patterns 4. There are a couple of theories, but we concentrate to most popular and major one. For at least million years of its geological history, Australia was embedded in the ancient southern super-continent of Gondwana.

At this time an inland sea covered central Australia, and silica-laden sediment deposited around its shoreline. It's believed that perhaps 30 million years ago, lots of silica was released into a solution which filled cracks in the rocks, layers in clay, and even some fossils.

Some of them became precious opals. It takes about 5 to 6 million years to make a 1 cm opal to mature. The evidence that Australia used to be under the sea is that many fossils of shells and fish bones have been found from inland of Australia, and some of those fossils were replaced by silica see picture below "Shell Opal". We already know that opal is made from silica combined with water. Some other components have been found inside opal, but we think these other components will just make it complicated in explaining.

Opal has been grown in jars using a electrolyte a chemical solution that is electrically charged which can takes weeks to see some colours. This is only a gel material and not hardened. This process is believed to happen in nature and it would explain how opal got inside some volcanic tuff or in the centre of a kernel.

Using an electron microscope, Dr Sanders found that opal was made up of millions of tiny silica balls in a regularly arranged pattern. In between each of these balls were found even smaller holes or interstices, through which light is diffracted, that is, when white light or ordinary sunlight shines through the holes, it is split into colours. In opal where the balls were small, the colours produced were the darker colours of the rainbow—violet, indigo and blue.

When the balls were large, yellow, orange and red colours were produced. Precious opal is found when the balls are in a regularly arranged pattern. In potch or common opal there is no set pattern. It seems that in different location opal was formed by many different influences to produce opal with a unique character from that field.

The author is a PhD volcanologist and author of the Gemmologica Italiana. Thanks to Marco Campos for permission to publish. Opal is a hardened silica gel with the formula Si In the living organisms that product organic opal, such as diatoms, radiolarians, sponges and many vascular plants, silicic acid accumulates in special cellular compartments where specific proteins induce the precipitation of opal-A.

The inorganic opal instead, originates from a combination of factors.