This quartz variety includes all hydrated silica varieties (*). It is transparent and used in jewel making. This stone is formed by the deposit of low-temperature siliceous waters. Opal is fragile: it can be degraded easily by salty and soapy solutions. In excessively dry locations it can dehydrate and crack.  The quality of Australian opal is excellent; in particular Queensland boulder opal. Boulder opal is very stable and is not affected by soap or dry weather. It is actually found in one of the driest place on earth, the arid outback Queensland. Compared to Mexican opal (top left photo), which is found in volcanic rock, Australian opal is extracted from a sedimentary rock (sandstone).  It is less fragile, and renowned for its stability, and is actually harder. (sic Jacky Delarue, miner)

(*) silica: Silicium oxide with formula SiO2. This stone is very hard, white and abundantly found in nature, as it constitutes more than half the Earth’s crust. Silica is the main component of sand.