Crystal Grimoire - Glossary

A through M


Accessory - A mineral usually associated with a given rock but always in small quantities

Accretion - the accumulation of sediment (mud, sand, etc.) by deposition, often occurring along a shoreline or in a river delta.

Acicular - bristly, spiny, needlelike. A thin, rather elongated crystal.

Acidic - adjective used to describe magmas that contain more than 60 percent SiO2 and also to igneous rocks and masses solidified from such magmas.

Acroite - a rare, colorless variety of tourmaline.

Adamantine - having a luster like that of a diamond.

Alloy - An alloy is a combination of two or more metals.

Amorphous - means without form. Examples are jet, amber, or ivory, they do not have a regular internal structure.

Assay - An assay is a test of the purity of an alloy. A tiny piece of metal is scraped from the piece and the percentage of gold or silver is determined.

Asterism- An asterism is a star-like luminous effect that reflects light in certain gemstones such as a Star Sapphire.

Axis - A straight line through the center of a plane figure or solid, around which the parts are symmetrically arranged

Axis of symmetry- An axis of symmetry (also called a rotational axis) is an imaginary line around which an object can be rotated a certain number of degrees and look like the original shape. When two planes of symmetry intersect, they form a straight line, which is an axis of symmetry.


Batholith- A batholith is a huge body of igneous rock that solidified under that Earth but now has at least 100 square kilometers (40 square miles) exposed.

Butte- A butte is a flat-topped rock or hill formation with steep sides.


Cabochon - Any precious stone cut in a convex shape, polished but not faceted.

Caldera - A caldera is a large depression formed from a collapsed volcano. Calderas are often circular or elliptical.

Carat - A carat (ct.) is a standard measure of weight used for gemstones. One carat weighs 0.2 gram (1/5 of a gram or 0.0007 ounce). A hundredth of a carat is called a point. The carat unit was introduced in 1907.

Chalcedony - Chalcedony is a family of minerals (microcrystalline quartz) that are often milky to gray to bluish in color. Chalcedony is porous and translucent.

Chert - Chert is a hard, dense, fine-grained type of sedimentary rock, a crystalline aggregate of silica (quartz). It was formed from deposits of silica-based skeletons of microscopic marine organisms. Black chert is called flint.

Chrysoprase - This is the most valued variety of the mineral chalcedony (microcrystalline quartz) that contains nickel, giving it an apple-green color. Chrysoprase is porous and translucent.

Clarity - Clarity is the clearness of a gemstone, or the lack of internal flaws. The clarity scale for diamonds runs from FL (flawless, with neither internal nor external flaws), to I3 (having many clearly visible imperfections using only the naked eye).

Cleavage - In minerals, it is the direction that a crystal usually breaks to produce a flat surface. There may be one or several cleavage directions.
In rocks, the direction in which certain metamorphic rocks, particularly slate, split.

Composite volcano- A composite volcano is a volcano that has a steep volcanic cone that is built up by dense lava flows and pyroclastic debris.

Conchoidal - "Shell-like," with a smooth curved surface.

Continental crust - Continental crust is the part of the Earth's crust that makes up the continents; it is roughly 20 to 75 km thick.

Continental shelf - The continental shelf is the part of the ocean floor next to each of the continents.

Crater - A crater is a circular depression in the ground. It has steep sides and contains a volcanic vent.

Crystal  - A crystal is the solid form of a substance in which the atoms or molecules are arranged in a definite, repeating pattern. The formation results in one of three ways: a liquid may cool or freeze into a solid; dissolved matter may precipitate out of a solution; or gases may condense into a solid form.

Crystal habit - Crystal habit is the crystal form that a particular crystal has. The most common crystal habits are:

Prismatic - elongated with parallel sides, like emerald, tourmaline

Tabular - short and flat (table-like), like morganite

Ocatahedral - eight faces, like diamond

Dodecahedral - 12 faces, like garnet

Acicular - needle-shaped, like rutilated quartz

Platy - occurring in very thin plates, like hematite

Crystallization - Crystallization is the process in which magma solidifies into solid, crystalline rock.


Density - Density is the weight per unit volume of a material

Diatreme - A diatreme is a volcanic vent or pipe that is formed by gas-charged magma.

Dichroism - Dichroism is the property of having more than one color, especially when viewed from different angles.

Druze - Druze is a layer of crystals that form within a mineral crust, like the inner cavity of a geode.

Ductile - A ductile substance is easily pulled or stretched into a thin wire. gold is the most ductile metal.


Earth's crust - Uppermost part of the earth. It varies from 30 to 70 kilometres beneath the continents, to only 6 to 8 kilometres beneath the oceans

Earth's mantle - The part of the earth lying between the core and the crust. It is some 2,900 kilometres thick, the upper part being mainly composed of olivine

Epoch - An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years.


Facet - A facet is one of the flat surfaces of a cut stone or glass.

Flaw - A flaw is a an imperfection in a gemstone. Flaws include: cracks, inclusions of other minerals or liquid-filled cavities.

Fluorescence - Fluorescence is property in which light (or other radiation) is emitted from an object.

Fossil - Fossils are the remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities.

Fracture - A fracture is a crack in a stone or gemstone

Fumarole - A fumarole is a vent from which volcanic gases (like sulphur vapor) escape.


Gemstone - A gemstone (also called a precious stone) is a mineral that is valuable, rare and often beautiful.

Geode - A geode is a rock whose crystal-filled interior can be hollow or filled.

Gneiss - Gneiss is a coarse-grained type of metamorphic rock that usually has light and dark bands (the bands are formed by the recrystallization of quartz, feldspars, micas and amphiboles).

Grain - A grain is a unit of weight used for diamonds and natural pearls. Four grains are equal to one carat.

Granular - Containing or consisting of grains.


Hackly - Sharp, jagged surfaces.

Hardness - Hardness of a mineral is a measure of its resistance to being scratched. (see Mohr's scale)

Heat treatment - Heat treatment is the heating of stones to a high temperature in order to enhance the color or clarity.

Hexagonal - These crystals are usually shaped like six-sided prisms or pyramids. Each crystal has four axes of symmetry. Three lie in the same plane, are the same length, and intersect at 120° angles. The fourth axis is not the same length, and is perpendicular to other three.


Icosahedron - Twenty sided polyhedron with all sides are equidimensional. Minerals shaped as icosahedrons belong to the isometric system.

Igneous intrusion - An igneous intrusion (also called a laccolith or a plutonic formation) is a formation in which magma (molten rock) is trapped beneath the surface of the Earth and pushes the rock located above it into a dome shape.

Igneous rock - Rocks which were once molten (magma)

Inclusion - An inclusion is a particle of foreign matter contained within a mineral. Inclusions can be solid, liquid, or gaseous.

Intrusions - Individual masses of igneous rock formed when magma works its way in between rocks and solidifies before it reaches the surface.

Irradiation - Irradiation is the act of being exposed to radiation. Many stones (like kunzite) are irradiated in order to enhance their color.

Isometric - Also called the cubic crystal system. Crystals can form in the shape of a cube (like halite and pyrite), an octahedron (diamond), dodecahedron (garnet), trapezohedron (garnet), and others. The most common and recognizable in this class are the cube, the octahedron, and the dodecahedron. Isometric crystals have three axes of symmetry, all at right angles to each other, and all of the same length.


Japanese twin - Form of contact twinning where two single Quartz crystals are joined by the their base at an angle near 90°.


Karat - Karat (abbreviated Kt) is a measure of the fineness of gold. 24 karat gold is pure gold. 18 karat gold is 18/24 gold (about 75% gold - three quarters gold). 14 karat gold is 14/24 gold (about 58% gold - a little over half gold). 12 karat gold is exactly half gold. 10 karat gold is 10/24 gold (only about 43.5% gold - less than half gold).

Kilogram - A kilogram (kg) is a unit of mass defined as the weight of one liter of water. One kilogram is equivalent to 1,000 grams or 2.2 pounds.


Lapidary - A lapidary is someone who cuts and polishes gemstones.

Lava - Molten rock with has poured out at the surface of the earth.

Liquid crystals: (LCs) are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. Liquid crystals can be divided into thermotropic, lyotropic and metallotropic phases. Examples of liquid crystals can be found both in the natural world and in technological applications.

Lithified - Lithified means turned to stone


Mafic - Mafic refers to dark-colored igneous rock or magma that have significant amounts of iron and magnesium.

Magma - Molten Rock inside the earth. Solidifies to form igneous rock

Matrix - Rock in which fossils or crystals are held. Also the finer materials between the pebbles etc of a conglomerate or breccia

Melee - A melee is a small diamond, under .20 carat.

Metamorphic rock - Any rock formed by the action of heat, pressure, or permeation by other substances on pre-existing rock material.

Microcrystalline - Microcrystalline is a type of mineral structure in which the crystals are so small that they can only be seen using a microscope.

Mohs' scale - Series of ten minerals arranged in order of increasing hardness against which the relative hardness of other minerals can be measured.

Monoclinic - Crystals are short and stubby with tilted faces at each end. Each crystal has three unequal axes. Two axes lie in the same plane at right angles to each other. The third axis is inclined.

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                                                                           Glossary: N through Z