In this amazing close-up we see a beautiful opal from Yoawh, Queensland, Australia. The length of this polished opal is a mere 20 mm but its features are gorgeous. 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.
To learn more about opals, the different varieties and sources etc. check out the extensive article on Wikipedia.