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How is opal formed?
Question Date: 2015-10-01
Answer 1:

Opal is a very cool mineral. Opal is made of silica (this is the stuff that makes up sand like you find on the beach) and water. Opal forms in lots of places, so long as they have enough water for it to form. The most common place for it to form is in tiny spaces (we call these pores) in other rocks. What happens is that water circulates through rocks in the crust, and if there is enough silica around, the water and silica mix together to form opal. This means that usually opal only forms in small amounts, but in some places there are perfect conditions for it to form, and the opal gets big enough to be sold to gem stores.

Opal most commonly forms in the pores of volcanic rocks.

Answer 2:

Opal is silica (glass) with inclusions of water inside of it. It is formed by a number of processes, some of them by living things that make tiny shells or pieces of opal inside of their bodies. I don't know how precious opal* is formed - the pieces of opal that living things make are so small that you need a microscope to really see them.

*This picture is from Wikipedia . Precious opal consists of spheres of silicon dioxide molecules arranged in regular, closely packed planes.

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