When you are choosing Opal Jewellery you will need to consider the gem’s base colour, brilliance, diffracted colour, pattern, shape and size.
With Black Opal or Black Boulder Opal, the degree of darkness in the body colour is taken into consideration. Usually the blacker the body colour the more valuable the gem.
In general, Black Opal is more rare, so the price is higher than Light (White) Opal.
Light opal is more desirable when it has a degree of transparency and if lively brilliant colours are present in Crystal Opals, they are most highly priced.
Brilliance, Diffracted Colour and Patterns
An Opal’s brilliance is very important, also how vibrant the colour of Opal is will determine its value. A good combination of brilliance and colour have an enormous impact on the value.
Opal is composed of silica spheres which are packed together. In precious Opal the arrangement of the spheres is an orderly three dimensional grid, because of the special pattern of spheres it is the only gemstone that has the unique natural ability to diffract light.
The colour visible from the Opal is dependent on the size of the spheres. The larger spheres create red and orange colours and blue and violet colours are created by spheres that are smaller.
Generally more colours in Opal is more valuable. Especially for Black Opal, the colours with red is the most valuable because red is the rarest colour, followed by orange, yellow, green, blue and violet. However a stone with dull red will be less valuable than a stone with vibrant blue/green colour.
There are many fascinating beautiful patterns, large patterns are more valuable than small pattern.
Shape & Size
Opal may be cut into many shapes and sizes. Opal shapes include oval, round, pear, marquise, square, rectangular, heart and etc.
Most popular way of cutting Opal is called "Cabochon" which has round dome shape, and it will be more valuable than a similar stone with a flat surface.
Sometimes irregular or asymmetrical shape of finer quality Opals can be made into an exclusive one-of-a-kind piece of jewellery.
As with other gemstones, Opals are measured in Carats. When all other elements are consistent, larger solid stones will be most valuable. However we have to be careful about the size of Black and Boulder Opal because they sometimes have different backings. The Opal traders usually measure the Opal by carat (one carat equals to 0.2g in weight), but also the actual size in millimetres of the stone is important too.