There are four types of opal; boulder, crystal, white, and black. Boulder opal is found in Australia, in cracks and crevices of brown ironstone boulders. Boulder opal is very low in water, so the gem rarely cracks or crazes. Crystal opal is pure opal and is transparent with vibrant flashes of color. White opal is the most common type of opal, it has a creamy white appearance with flashes of color. Black opal is primarily found in the Lightening Ridge mines in Australia. Black opal is the most desired and valuable, with breathtaking color flashes against the dark base color.
Opal is rated a 5.5 – 6 on the Mohs scale. It is a soft gemstone and must be worn with care.