Opal is a form of silica similar to quartz but containing water within the mineral structure. Precious opal consists of small silica spheres arranged in regular patterns. The colour in precious stones is caused by the diffraction of white light breaking into virtually all the colours of the rainbow. This play of colour depends on the angle of incidence of the light and can change when the gem is turned. The size and spacing of the spheres also controls the colour range of this precious gem.
Where do they come from?
Over 95% of the world’s supply is found in Australia. The town of Coober Pedy in South Australia is a major source. Another Australian town, Lightning Ridge in New South Wales, is where most black opals are found. It takes its name from the predominantly dark background they have. This darkness enhances the vibrant play of colour within the stone.
Whether an opal is ‘black’ or not can be determined by ignoring all colours and concentrating only on the overall body tone (background blackness level) of the opal.
Boulder opal gets its name from the fact that an ironstone backing is part of its natural formation. The colours can also be extremely bright and vivid. Gem quality ‘boulder opal’ can be very expensive competing in price with some of the black variety.
Fire opals are yet another type of this gemstone. They tend to be transparent and are found with a warm body colour ranging from yellow to orange. They do not have the ‘play of colour’ associated with other types of opal and look completely different. The most famous source of fire opals is Mexico. Hence these opals are popularly known as ‘Mexican fire opals’.
In 1974 a Frenchman by the name of Pierre Gilson discovered a way of creating lovely blue opals with all the properties of a natural opal apart from the presence of water. The process takes over 12 months. The advantage of Gilson’s variety is that it tends not to crack in the way that natural opal can when it loses its water due to extreme temperature changes. The Japanese have also ‘cultured’ the most beautiful fiery, blue opals using a form of quartz silica and zirconium oxide. They are initially formed in much the same way as Gilson’s but are then impregnated with polymer as a stabilizer ensuring that the silica structure is completely cemented together and the vivid play of colour in is permanent. This created opal gem has fiery blue colours and flashes of turquoise which are truly breathtaking!
How do you value opals?
The most important factor affecting the price of an Opal is the ‘play of colour’, the actual colours themselves and the pattern of colour. Flashes of red will enhance the value as this means most other colours will be apparent. For evaluating Opals the thickness of the stone is considered, the appeal of the pattern of colours, the cut, the weight and the finish. The following factors should all be taken into consideration:
- The play of colours in the opal.
- The brightness of the stone.
- The body tone of the gem.
- The saturation and intensity of the colours..
- Whether the opal appears dull when viewed from different angles.
- Whether it is flat on top or has a rounded dome.
- The size and weight.
- The cut and shape.
How should I look after my opal jewellery?
- Avoid extreme changes in temperature that could cause opals in your jewellery to crack.
- Avoid knocking or scratching opal jewellery. Remove your jewellery when playing sport or gardening. Never wear your jewellery when carrying out household tasks like washing dishes or using chemicals or cleaning agents.
- Clean your opal jewellery with a gentle detergent in warm water using a soft cloth or brush. After cleaning your opal jewellery should be rinsed in clean water to remove any residue.