Sapphire is the name for the mineral
corundum when it is blue in color. Minerals
are composed of atoms of certain elements in
certain arrangements. Corundum is made of
aluminum and oxygen, and can be blue when the
element titanium is incorporated into its
atomic structure. Corundum, and therefore
sapphire, forms in coarse grained igneous rocks
that do not have much silica (silicon and oxygen).
Coarse grained igneous rocks are formed when
molten rock cools slowly under the surface of the
earth. Slow cooling allows large crystals of
minerals to grow. So the slower the magma
cools, the larger the sapphire that forms.
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