Pegmatites are exceptionally coarse-grained rocks born of molten rock, or magma, beneath the Earth's surface. They produce many of the crystal giants of the mineral kingdom. Over 300 minerals come from pegmatite deposits. Some of these minerals are extremely rare, form exquisite gem-quality crystals, are found only in pegmatites, provide important industrial materials. As molten rock cools underground, the water-rich portion crystallizes last. It concentrates rare elements that did not fit into earlier-formed minerals. This enriched magma squirts into cracks in surrounding rock and cools.