Opal is a non-crystalline form of silica; because of its amorphous character it is considered to be a mineraloid unlike other forms of mineral silica, like quartz. The many colors found in opals are created by a diffraction of light caused by the opal’s internal structure - because of this, opals have the ability to represent every color in the visible light spectrum.
The vast majority of opals are found in Australia, although in 2008 NASA reported opal deposits were discovered on Mars in dissolved basaltic rock.
This brings me all to my absolute favorite item on public display at The Field Museum: the opalized vertebrae of a plesiosaur. As if bone couldn’t be beautiful enough on its own, it went ahead and mineralized itself with silica to put all other fossils remnants to shame. You can see it on display in the Grainger Hall of Gems.
Sources  
my birthstone is the prettiest.