Check out this gorgeous close up of opal from Yoawh, Queensland, Australia. The length of this polished opal is a mere 20 mm. The opal is a part of the Willems Miner Collection. Opal is the national gemstone of Australia.
According to Wikipedia, an opal is:
a hydrated amorphous form of silica; 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 the other 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.