Australian opals are world-renowned for their beauty and visual interest. The areas where many of these opals are mined is such an arid desert that entire mining communities are built underground--often with all the amenities of normal housing--to avoid the desert's intense heat. Boulder Opals are a special kind of Australian Opal, where the relatively fragile opal material is held in sturdy matrix, which not only protects the stone, but also adds character to each piece. That's why these stones are also called "Opals in Matrix." Koroit Opals and Yohawah Opals are two of the more well-known boulder opals from Australia.