Descriptionary

diamond a pure carbon crystal, the hardest mineral known, used in jewelry and for industrial cutting and abrading. Clear crystal diamonds are the most com­mon, but they can also be blue, yellow, green, black, white, pink, and violet.

diatomite a sedimentary rock formed by nano- plankton.

dolomite a sedimentary rock with a high concentra­tion of calcium-magnesium carbonate.

drift sand, gravel, and rocks deposited by a glacier.

drumlin an elongated hill of compacted rocks and gravel deposited by a glacier.

druse a layer of quartz crystals that form on another stone and especially inside a geode. Also known as drusy.

emerald a valuable green gemstone in the beryl family.

erratics rocks or boulders transported by glaciers that differ from native rocks.

esker a long, winding ridge of sand and gravel deposited by a glacial stream.

extrusive of magma, ejected out onto the Earth's surface.

fault a break or large crack in a continuous rock formation.

feldspar a common group of hard crystal miner­als including calcium, sodium, potassium, and other materials.

flint a very hard form of silica, best known as a spark producer when struck against steel.

fold a bend in rock strata.

fool's gold popular name for pyrite, a bright, metal­lic, brassy yellow rock often mistaken for gold.

frost agate a form of agate with white, frostlike marks.

fuchsite a glassy, deep green variety of muscovite.

garnet a group of minerals, the most popular of which is pyrope. Often, a red, semiprecious crystal resembling a pomegranate seed, but garnets come in all colors except blue.

gastrolith any stone ingested by an animal to aid in digestion.

geode a globular rock with a cavity lined with min­erals.

glacial pavement rock paved over by glaciers, leav­ing telltale scrapes, scars, and gouges, or a polished appearance.

gneiss a common metamorphic rock similar to granite.

granite a hard igneous rock composed of quartz, feldspar, and mica; a popular building material.

greenstone an alternate name for nephrite, a variety of jade.

heliotrope see bloodstone.

herkimer diamonds a clear form of crystal quartz, not an actual diamond.

hornblende a green or black mineral, found in igne­ous rock, and one of the components of granite.

humus the dark, organic matter found in soil.

hyacinth opal a yellow or orange form of opal. Also known as girasol.

igneous rock a class of rocks formed from cooled magma or lava.

imperial jade an emerald-green form of jade.

imperial topaz a very valuable form of orange- yellow topaz.

intrusive of or relating to igneous rock or magma that forces its way into or between other rocks and solidifies before reaching the Earth's surface.

ironstone a heavy, sedimentary rock with a high iron content.

jacinth a red, translucent zircon used as a gemstone.

jade a very hard stone, composed of two differ­ent minerals, jadeite and nephrite, and highly prized for its beauty. Although it comes in different colors, the green variety is most sought after, especially for jewelry.

jasper an opaque quartz that may be red, yellow, brown, or green, with swirls and bands of colors.

karst a topography of limestone characterized by numerous sinkholes or caverns.

lapidary one who cuts and polishes gemstones.

lapilli small volcanic fragments from pebble to cob­ble size.

lapis lazuli a semiprecious stone characterized by its royal blue color interspersed with gold flakes of pyrite and white streaks of calcite.

limestone a sedimentary rock composed of calcium carbonate from deposits of various marine creatures.

lithification the process of rock formation.

loess deposits of windblown silt.

mabble a metamorphic rock, often white and swirled with various colors.

malachite an opaque, semiprecious stone, usually greenish because of its high copper content.

marble metamorphosed limestone.

marbled having swirls of colors, like marble.

matrix the natural rock in which a gemstone is embedded.

melanite a black garnet.

metamorphic rock rock that has been "metamor­phosed" or altered by heat, steam, or pressure to form other types of rocks.

Mexican diamond not an actual diamond but rock crystal.

mica a soft, transparent mineral that forms in sheets that can be peeled or flaked off, and which can appear in various colors.

Mohs scale a scale, from 1 to 10, designating in gemstones the degree of their hardness and resistance to scratches, with diamonds being the hardest of all and rated a 10. A diamond can scratch all other gems, but it cannot itself be scratched.

moldavite a rare green and glassy gemstone formed by meteorite impacts.