蛋白石，又名闪山云，英语音译为“Opal”或“澳宝”，宝石学名称为欧珀，是二氧化硅的水合物，成分为SiO₂·n H₂O，是非晶质结构，所以无一定的外形，断口为贝壳状，主要是二氧化硅的胶体沉淀形成的，如果沉淀在生物遗骸中，则形成“树化玉”。

蛋白石的含水量并不固定，一般在3%-10%左右，但也有高达20%的；硬度为5-5.5；比重为1.9-2.5；一般为蛋白色，如果有其他原子混入，可以形成各种颜色，例如含铁、钙、镁、铜等，蛋白石一般具有玻璃光泽或蜡状光泽，如果出现色彩光泽随角度变化，则是贵重的宝石，否则只是装饰性石材。

Opal is a hydrated amorphous form of silica (SiO₂·nH₂O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Opal is the national gemstone of Australia.