|How Gemstones are Formed
Minerals form in many different environments in the Earth. Most gemstones form in the earth's crust, the top layer of the earth, with a depth of 3 to 25 miles. Only two gemstone varieties -- diamond and peridot -- form in the earth's mantle, which represents 80% of the earth's volume. The mantle consists mostly of melted rock called magma with a solid upper layer.
While a few gemstones originally formed in the mantle, all gems are mined in the crust. The crust is made up of three kinds of rocks, known in geology as igneous, metamorphic and sedimentary. These technical terms refer to the way in which rocks formed. Some gemstones are associated especially with one kind of rock; others with multiple types.
The igneous process involves the solidification of magma. Magma from the mantle can rise to the crust, usually through volcanic pipes. If it reaches the surface of the earth, it solidifies as lava. But if the magmatic mass cools slowly in the crust, it can crystallize and form minerals. Increases in pressure can also cause this pegmatitic fluid to infiltrate surrounding rocks, often making chemical exchanges with them. The long list of gemstones formed from igneous rock include the chrysoberyl group, all of the quartzes (including amethyst, citrine and ametrine), the beryls (emerald, morganite and aquamarine), the garnets, moonstone, apatite, diamond, spinel, tanzanite, tourmaline, topaz and zircon.
Once the igneous rock reaches the surface of the earth, the forces of erosion and weathering produce smaller particles which accumulate on the surface or are moved by wind and water. As time passes, layers of these sediments build up on land or under water. The pressure from upper layers causes compaction in the lower layers along with various chemical and physical changes such as lithification, which lead to the creation of sedimentary rock. Evaporation is an another process which also produces sedimentary rocks, as when dripping mineral-laden waters leave behind stalactites or stalagmites. Gemstones associated with sedimentary rock include jasper, malachite, opal and zircon.
The presence of intrusive magma in an area (known as contact metamorphism), or of tectonic plate interactions on a larger scale (known as regional metamorphism) puts the igneous and sedimentary rocks and minerals under heat or pressure which may cause changes in their chemistry and crystal structure. The result is the creation of metamorphic rocks. Gemstones associated with metamorphic rock include the beryls, jade, lapis lazuli, turquoise, spinel, ruby, sapphire and zircon.
Rocks and minerals are in a constant state of change, referred to as The Rock Cycle. Igneous rock can change into sedimentary or metamorphic rock. Sedimentary rock can change into metamorphic or igneous rock. And metamorphic rock can change into igneous or sedimentary rock. But you have to be patient.
- First Published: July-22-2008
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