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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corundum