Opal's internal structure makes it diffract light; depending on the conditions in which it is formed it can take on many colors. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. The reds and black tones are the rarest type whereas white and greens are the most common. The stone also varies in density from opaque to semi transparent. The colours can be enhanced by using basalt.
There’s lots of different names for the different types of Opal, Dark or Black Opal, White or Light Opal, Milk or Crystal Opal, Boulder Opal, Opal Matrix, Yowah Nuts from Queensland and also Mexican and fire opal. Opal variations are practically unlimited.
They all show in their own special way that unique play of colours – except for Fire Opal, which is due to its transparency. If Opals are lacking the typical play of colours, they are simply named “Common Opal”. Precious opal shows a variable interplay of internal colours and does have an internal structure. At the micro scale precious opal is composed of hexagonal or cubic closely packed silica spheres.
Opal is a form of quartz but because opal is actually a gel, it is, strictly speaking, not a form of quartz. Quartz is a crystalline form of silicon dioxide, opal is a solid gel. The hardness of opal is 5.5 to 6.5. therefore they appreciate a protective setting. In earlier days Opal’s sensitive surface was often oiled, but today also sealing them with colourless artificial resin has become quite popular.
Opal doublets, often used in jewelry, are thin slices of precious opal glued onto a base material. Such gems are considerably cheaper than solid opals, yet provide the same play of color. Opal doublets are sometimes coated with a thin layer or dome of clear Quartz to make them more resistant to scratches (since Opal is a relatively soft gem). These are sometimes called Opal triplets.
In order to bring out the best play of colour in Opal, the stones are cut and polished to round or oval cabochons, or any other softly domed shape depending on the raw material. Only the best qualities of Opal, however, are suited to faceting. The Opal cutter will first of all carefully remove any impurities using a diamond cutting wheel, before working out the rough basic shape. This comes the fine cutting, the finishing with sandpaper and then the final polishing with a wet leather wheel.
Due to the differing percentage of water, Opals may easily become brittle. They always contain water – usually between 2 and 6 per cent, but sometimes even more. Thus if stored too dry or exposed to heat over a longer period of time, Opals will show fissures and the play of colour will become paler. Therefore, Opal jewellery should be worn as often as possible, for then the gemstone will receive the needed humidity from the air and from the skin of its wearer.
In my next article I will discuss the origin and history of the Opal.