The Queen of Gems
What is Opal?
Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal generally contains 2-8% water and consists of small silica spheres arranged in a regular pattern. Opal occurs in many varieties, two of which are precious opal and potch (common opal with no colour play).
Colour play in precious opal is caused by the regular array of silica spheres and voids diffracting white light, and breaking it up into the colours of the spectrum. Much like the water particles do when creating a rainbow. The diameter and spacing of the spheres controls the colour range of an opal. Small spheres (approx. 150-200 nm; I nm = 1 0’9 m) produce opal of blue colour only, whereas larger spheres (350 nm) produce red colour. Opal with red colour can display the entire spectrum. Opal colours also depend on the angle of light incidence and can change or disappear when the gem is rotated.
Geology of Opal
All precious opal in Australia occurs in rocks affected by weathering during the Tertiary Period (1.8-100 million years ago). The weathering process broke down minerals of the country rock to produce kaolin (a clay) and soluble silica. It also created cavities in the rock by dissolving soluble minerals and fossil shells. These cavities, together with faults and fractures in the rock, provided pathways for underground water containing the soluble silica. Periodic lowering of the water table, possibly caused by changes in climate, carried silica-rich solutions downwards to deposit opal in the rock cavities.
Value and Presentation
Attempts have been made to establish guidelines for determining opal prices but they have been largely unsuccessful because of the gem’s infinite variations. For an estimate valuing of opal, please read more on our Valuing Australian Opal page here
Solid opal is a 100% natural opal that has been cut and polished only, the colour-play and body tone (base colour) are exactly as it was found in its natural state. Usually, solid opals are cut ‘en cabochon’ (rounded) as this highlights the colour-play and is most suited to setting in jewellery. Solid opal can be Full Colour, which means it shows play of colour in the entire stone. Layered Colour, where the play of colour appears in a band or bar, usually on colourless opal such as the majority of black opal or ironstone in the case of boulder opal. These are the only two varieties of opal we recommend.
Opal can also be Matrix Colour where the opal is inpregnated into host rock such as Matrix opal and fairy opal, this is the least expensive and most often treated or altered to show its play of colour. Always look for solid opal as this is true opal and has not been altered in any way from its natural state other than shaping and polishing.