Opal is a spectacular gemstone which is unlike any other.
It is a form of silica that contains up to 20% water. It is found in every color imaginable and usually has multiple colors in the same stone. It is with good reason that the ancient Romans considered this the most precious of all gems!
The flashes of different colors in precious opal are called “play-of-color”. These are the result of the interaction of light with the microscopic spheres of silica in the structure of the gem. The brighter and more multicolor the play-of-color, the more valuable the opal. Opals occur in a variety of base colors and transparencies. White, gray, chocolate brown and black base colors are the most common. Certain patterns and combinations of color are more desirable than others, and the best opals should show strong color from all directions when the stone is moved around. One of the wonderful aspects of opal is that no two opals are alike.
The traditional source and largest producer of opal is Australia. Recently however, precious opal was discovered in Ethiopia, in East Africa. These are called Welo (sometimes called Wello or Wollo) opals, from the name of the area where they are found. These African opals are quite different to the Australian stones. Unlike Australian stones which occur in sedimentary rock, Ethiopian opals are found in igneous (volcanic) rock. They are also an unusual type of opal called hydrophane. Hydrophane opal has the fascinating property of being able to absorb and release water. When a dry hydrophane opal is exposed to water over a few minutes it will absorb some and become more transparent, sometimes changing the appearance of the stone quite dramatically. This process will reverse itself as the stone dries out again. Sometimes even changes in humidity can make the stone look different!
Welo opals are known for their exceptional brilliance and fantastic patterns. Their base color varies from white to brown to black and they may be opaque to transparent. Welo opal is sometimes treated with smoke to darken the base color. These smoke-treated stones have a similar value to the white gems and the treatment is permanent and stable. Ethiopian opal has been found to be more durable than Australian opal, however it still requires some special care. Opal has a hardness of approximately 5-6 on the Mohs Scale and is thus better suited to pendants and earrings. It can be used in rings if care is taken, ideally with a setting that offers some protection to the stone. Opal should not be exposed to rapid temperature changes such as hot water, exposed to harsh chemicals or cleaned in an ultrasonic cleaner.
Opal is the traditional birthstone for October, along with tourmaline.