All rocks fall into one of three classifications, igneous, sedimentary, or metamorphic. Igneous form from the cooling and crystallization of molten magma. Sedimentary rocks are composed of materials that result from mechanical and chemical weathering of preexisting rocks. Sediment is deposited in layers become rock when cemented or compacted together. The change is in a solid state, caused by high heat and pressure resulting from burial deep within the crust of from intrusive igneous rock.
Gems can also form from solution as vein or secondary minerals.
The Earth is at least 4.5 billion years old, beginning as a hot, fiery sphere of molten material. As the molten mass cooled, crystals formed and the Earth's surface was covered by a layer of igneous rock. Most igneous rock is made up of quartz, feldspar, nepheline, mica, amphibole, pyroxene, and olivine. The texture of the grains and the relative amounts of these minerals help to classify the rock. In addition to the minerals used for classification, igneous rock contains other minerals, called accessory minerals.