Diamonds form in igneous rocks called kimberlites.
Mineral crystals form in many ways throughout the world. You can find crystals in sedimentary, metamorphic and igneous rock environments. Unique formations in geodes and caves include crystals of many colors, shapes and sizes. The environment in which a crystal forms tells of the special history of that part of the Earth.
Cave Crystals
Crystals, such as calcite, form in caves through precipitation. Caves form when rain and carbon dioxide mix to form an acid that dissolves underground limestone. As the limestone dissolves, it leaves open space for crystals to form. Calcite is the main mineral in limestone. Calcite crystals will form as the calcite-rich waters in the cave precipitate in the open space and form solid crystals. Cave crystals can form in many sizes and may be grouped together or form separate, perfect crystals.
Geode Crystals
Geodes are round or oblong rocks usually filled with crystals of quartz. Other varieties of quartz, such as amethyst and citrine, are also common in geodes. Geodes often form in open areas of sedimentary and igneous rocks. These open areas fill with silica-rich water that precipitates quartz in the open space. Often, layers of quartz agate initially line the opening. The agate may completely fill the void or large crystals will form on top of the agate, leaving some open space inside the geode.
Evaporite Crystals
Minerals, such as gypsum and halite, form from the evaporation of enclosed salt-water bodies such as lakes. As the water evaporates, crystals form in the lake bed. These crystals may form in wide, thick layers over the entire size of the lake.
Igneous Rock Formation
Medium to large mineral crystals form in intrusive igneous rocks. The magma from deep within the Earth moves upward into the surrounding rock and cools very slowly. This magma will form different types of igneous rocks based on the composition of the magma. The slow rate of cooling allows sufficient time for medium to large sized crystals to form in the rocks. Quartz, diamond, mica and feldspar are common igneous rock mineral crystals.
Metamorphic Rock Formation
Some mineral crystals form from metamorphic processes. Metamorphic rocks form when high heat and pressure changes one rock into another. These conditions occur as a rock is buried or moves close to a heat source. During this process, new mineral crystals may form within the rock. Kyanite, garnet and staurolite are common metamorphic crystals.
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