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Mineral gemstones form through igneous, metamorphic and sedimentary
processes. Many gemstones are found in pegmatites which is a vein
of concentrated minerals. It usually forms when magma cools inside the
Earth’s crust (plutonic). Elements and compounds become
concentrated and "squeezed" through cracks of the surrounding
cooled rock. Emerald, ruby, gold and silver and many other gems are
produced in this way. In contrast, jadeite a beautiful green rock
gemstone, and most garnets, are produced by metamorphism (high
pressure and temperature).
Sedimentary rocks can also produce gemstones through precipitation.
Many gemstones composed of quartz, such as amethyst, jasper, agate, and
tiger-eye are examples. They form by water circulating just below the
surface of the Earth with high concentration of dissolved silica. When
the water passes through gaps in the rock, the silica precipitates
slowly (hundreds to thousands of years) to form quartz minerals.
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Pegmatite with garnets from
San Diego
Agate
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