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.
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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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