SOLIDS, DOUBLETS AND TRIPLETS
Australian opals are the finest in the world, but before you race out and buy one, learn a little about them – what form they come in and what’s available.
They fall into three categories: solids, doublets and triplets.
Solids are natural stones which have been cut and polished. Many of them have natural potch or rock backs.
Doublets are two pieces cemented together. Bonding a layer of high grade opal to a black back enhances the colour and makes them very attractive. Some have a nice cabochon top, which in many cases, resembles black opal.
Triplets are doublets that are usually made from a wafer-thin layer of opal with clear domed caps cemented to their faces. The cap can either be glass or quartz, depending on the quality of the opal. Not only does it protect the opal, but also magnifies the intensity of the colour and pattern, greatly enhancing the overall appearance of the stone.
Boulder opal in its many forms is usually cut into solids.
FACTS AND MYTHS
A few facts and myths about your beautiful opal:
The most important thing when viewing opal is the angle at which light falls on the gem. To see opal at its best the light must come directly over your shoulder. When using natural light inside, always stand with your back to a window or an open door. The same applies with artificial lighting. Reverse these positions and you will be amazed how the colours change. Most jewellery stores use this overhead light technique to display their goods.
One of the more common myths is that oil or common household liquids will damage your opal. There is no known compounds that can penetrate a solid Australian opal. If there were, there would be very few light opals available, as most would be doctored with some form of dye to darken and enhance their beauty.
Another myth is that you should occasionally soak your opal in water. Again, if it’s Australian opal, you’re wasting your time, but if you do, you won’t damage or enhance it. So far opal has proven to be impervious to all enhancing liquids, so you can buy without fear of them having been doctored like some other well-known gems.
The best way to preserve your opal for future generations is to wear, handle and enjoy it. Strange as it may seem, this beautiful gem of passion loves it.
OPAL MICROSTRUCTURE AND GEOLOGY
Opal derives its colours in the same way as the rainbow. It is made up of billions of closely packed spherical particles of amorphous silica, stacked in a three-dimensional grating. Depending on their size, when light passes through them it is diffracted into wavelengths of different colour. They are so small that two capped hands would hold more than all the grains of sand on Bondi Beach (Australia). It requires 20,000 of them to cross the smallest dot of a ballpoint pen.
Australian opal contains between 6% and 7% water. It requires 60 degrees Celsius to begin removing water from the first few layers of spheres and 250 degrees to reach the maximum rate of evaporation.
Most Australian opal is found in Cretaceous sediments. It has now become a matter of conjecture as to the method and time period in which it was deposited. The general model for many years has been that silica in surrounding rocks has been dissolved over millions of years, releasing it into the water at 120 ppm. This silica was then deposited in faults, fractures and cavities left by decomposed bones and other soluble materials.
Reproduced with permission from the booklet “Opals, The Beautiful Gem of Passion” by Len Cram, Kingswood Press, 2010