Opals (shown above) are 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.

The internal structure of precious opal makes it diffract light. Depending on the conditions in which it formed, it can take on many colors. The one pictured on the left is called the "Virgin Rainbow" opal, being the finest ...

This specimen of Millerite, housed at the Royal Ontario Museum, is one of the rarest examples in the world, with tiny, crystalline hairs that have grown from the initial rock.

Dozens of stones up to 300 kg in weight are traveling for hundreds of metres across a dried lake in America. But no one's ever seen them do it. So, what's making these Death Valley 'sailing stones' move?

This is what it looks like to swim between two continental plates. The Silfra fissure in Iceland separates the North American and Eurasian plates, which drift two cm away from each other every year, causing earthquakes about once per decade.