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. These minerals can form large, perfect crystal gems, such as zircon, sapphire, and pyrope garnets. After the rock weathers, the minerals are recovered from unconsolidated sands and gravels. Igneous rocks can be classified into intrusive, extrusive, hypabssal, or pegmatites environments.