It is an important coloring constituent of soil. Opal is a silicate lacking crystal structure that forms in association with silica infiltrating the ground, depositing opal in bone, wood and often associated with hot springs. Chalcedony is microcrystalline quartz that forms under the ground. Varieties of this include flint, agate and jasper. Silicate minerals commonly occurring in igneous rocks.
Metamorphic minerals form under pressure and often from remelting of the parent rock.
Talc forms as a secondary mineral by the alteration of other magnesium silicates. Staurolite, commonly forming cross-shaped twin crystals, occurs in regionally metamorphosed schists and gneisses. Serpentine is a secondary mineral resulting from the hot-water alteration of magnesium silicates. Kyanite forms in schists and gneisses fromed from rocks rich in clays. Kyanite is used in porcelain, bricks and spark plugs.