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Birthstones: Myth and History

Birthstones Glossary

Carbon: A chemical that is found in all plants and animals, as well as in some nonliving things such as rocks, coal, diamonds, and petroleum.

Crystal: A crystal is the external expression of the arrangement of atoms in a mineral. Crystals appear geometric in form because the atoms are organized geometrically within the mineral.

Druze: A druze is a layer of crystals that forms within a mineral crust, like the inner cavity of a geode. Amethyst crystals are often found in a druze.

Feldspar: A mineral composed largely of linked chain of silica, aluminum, and oxygen elements. Along with quartz, it is the most common mineral on the continents. The word means "field crystal."

Gem: A cut and polished mineral of beauty, durability, and rarity.

Geode: A rounded, irregularly shaped mass of stone having a cavity lined with crystals.

Gneiss: A banded or foliated metamorphic rock, usually of the same composition as a granite.

Granite: A light-colored, coarse-grained, igneous rock characterized by the minerals orthoclase feldspar and quartz, with lesser amounts of plagioclase feldspar and iron-magnesium minerals.

Igneous: Rock formed from the cooling of magma (molten rock), either above or below the ground.

Isometric system: Also called the cubic crystal system. Crystals can form in the shape of a cube (such as halite and pyrite), an octahedron (diamond), dodecahedron (garnet), trapezohedron (garnet), and others.

Lamellar: Composed of flat, thin layers.

Metamorphic: A rock changed from its original form and/or composition by heat, pressure, and chemically active fluids, deep within the crust of the Earth.

Mineral: A naturally occurring solid that has a well-defined chemical composition and in which atoms are arranged in an ordered fashion.

Olivine: A rock composed of silica and oxygen with varying amounts of magnesium and iron; brown, yellow, or green color. It is common in certain volcanic rocks.

Phosphate: The generic term for a compound containing a phosphate group; any of several phosphorous/oxygen combinations.

Quartz: A very hard, extremely common mineral composed of silica, SiO2, found worldwide in many different types of rocks, including sandstone and granite.

Rutile: A lustrous mineral that's red, brown, or black in color and is composed of titanium dioxide along with a little iron.

Schist: A metamorphic rock characterized by distinct layers of mineral grains (usually micas).

Silicate: Silicon and oxygen in the shape of a tetrahedron that is the heart of this most important of the rock-forming minerals. It has been estimated that up 95% of the Earth's crust is made up of silicate minerals.

Kimberlite is the ancient volcanic rock in which diamonds are found.