Absorption Spectrum - Colors of light least absorbed combining to produce the color of the stone. The stone, when viewed by spectroscope, will show as dark bands in characteristic positions the colors most strongly absorbed. 

Acicular - Needlelike; refers to the growth of a mineral in long and slender crystals. 

Adamantine - Very high luster. 

Aggregate - Intergrowth of several crystals, these may be globular, fibrous, reniform, or radiating fibrous. 

Adularescence - The sheen of color seen in moonstone and other feldspars of the adularia variety. 

Allochromatic Minerals - Minerals that are colorless when pure, the color coming from coloring agents, most of which are, cobalt, copper, chromium, titanium, vanadium, manganese, and iron. Examples of this are beryl, corundum, quartz, and spinel. 

Amorphous - Has no characteristic external form or shape. The arrangement of the atoms and molecules are irregular 

Amphiboles - A group of closely related, dark colored rock forming silicate minerals, as in, actinolite, hornblende. 

Anisotropic - when applied to crystals it is the display of unequal physical properties in different directions. An example would be a mineral which has a different hardness when tested in different directions. 

Anisotropic - Crystals in which the optical properties vary with direction. All crystals except those in the cubic system are in this category, and exhibit double refraction. 

Aphanitic rock - The rock in which the crystals are too small to be seen by the unaided eye. 

Asterism - Stones containing suitably oriented rod like inclusions or channels, that are cut as cabochon in the correct direction show this star effect. Star effect. 

Atomic Weight - Weight of an atom compared with an atom of oxygen (16.00). 

Biaxial - Two optic axes or double refraction. Usually crystals in the rhombic, monoclinic, and triclinic system. 

Birefringence - Same as double refraction. Splits rays of light passing through a transparent object as glass or crystal. 

Botryoidal - Resembling a bunch of grapes in rounded masses of a mineral. 

Brilliant - The cut of a gemstone that is round and has 32 facets plus the table above the girdle, (crown), and 24 facets plus any culet below the girdle, (pavilion).

Cabochon - The cut of a gemstone that has a convex surface. A cab. 

Cameo - (Italian) name for a relief that is cut so that it is raised

Chalcedony Species - Moh's hardness: 6.5 - 7
Chemical composition: SiO2, silicon dioxide
Crypto crystalline quartz (microscopically small crystals) generally known as chalcedony; includes agate, petrified wood, chrysoprase, bloodstone, jasper, carnelian, chalcedony, and sard.

Chatoyancy - Cat's eye effect produced by some gemstones when cut properly in cabochon. See asterism. 

Chelsea Filter - A dichromatic color filter transmitting light of only two wavelengths, one deep red the other yellow green. Used to discriminate between emerald and synth. spinel and green glass colored with cobalt. 

Cleavage - The tendency of stones to split along one or more definite directions, always parallel to a possible crystal face. 

Conchoidal - (fracture), A breakage which leaves a conchoidal shell shaped surface. 

Concretion - Knobby or rounded mineral concentrations in sedimentary rocks that are completely surrounded by rock. 

Crypto crystalline - The structure of a substance as chalcedony, that consist of very small crystals but show no external sign of crystal structure. 

Crystal - A homogeneous body in the form of a geometric solid bonded by polyhedral faces, the nature of which is expression of the orderly and periodic arrangement of its constituent atoms. 

Crystal Axes - "Lines" passing through a crystal in important symmetric directions, intersecting at the center of the crystal. 

Crystal Systems - The six main groups into which crystals can be classified: triclinic, monoclinic, orthorhombic, cubic, tetragonal and hexagonal. 

Dendrites - Skeletal crystals that develop from supersaturated solutions, often in small cracks, often resembling plant or trees. 

Density - The ratio of the weight of a substance to its volume expressed in g/cm 3, and numerically equal to the specific gravity. 

Diaphaneity - Showing light through its substance; transparent; translucent. 

Dichroism - Possessing the property of showing two different colors when viewed from different angles. 

Dispersion - The separation of white light into its constituent colors by its refraction or diffraction. 

Double Refraction - Ability of certain crystals to split incident light into two rays with different refractive indices. 

Dripstone - stalagmites or stalactites 

Druse - A crystal coated surface of rock. 

Enhydro - A variety of chalcedony or or quartz species having the center cavity filled with water. After removing the mineral from the surrounding rock, the water often dries out.

Facet - Man made flat part of a mineral, a planar surface. 

Ferromagnetism - magnetic even in the absence of an external magnetic field. 

Fire - Same as dispersion 

Fluorescence - a mineral's ability to glow in the presence of ultraviolet light. Some minerals will glow with characteristic colors in long wave or short wave UV light.

Foliated - Made up of thin leaves, like mica. 

Fracture - A break with an uneven or irregular surface.

Fraunhofer Lines - A series of groups of dark lines in the spectrum of an object, visible using a spectroscope.

Friable - Easily crumbled or pulverized. 

Geniculated - Knee like intergrowths of crystals. 

Geode - A stone having a cavity lined with crystals; the cavity in such a stone. 

Girdle - The wide part of a cut gemstone 

Gliding plane - a crystal direction along which the atoms can slip a defined distance without destroying the coherence of the crystal. 

Habit - Characteristic crystal form. 

Hacklt - A fracture characteristic of metals in rock, like gold and copper. (hackly) 

Hardness - a mineral's resistance to being scratched. The Mohs Hardness Scale is most often used. It runs from 1 to 10, 10 being the hardest. The scale is not linear, which is to say that hardness 10 is not twice as hard as hardness 5. Each hardness is represented by a particular mineral, some by a common material, and the actual hardness is indicated by the Rosiwal cutting hardness: 

H Mineral Common item Rosiwal cutting hardness
== ========== =========== ========================
1 talc 0.03
2 gypsum fingernail 1.25
3 calcite copper coin 4.5
4 fluorite 5.0
5 apatite knife 6.5
6 orthoclase 37
7 quartz steel file 120
8 topaz 175
9 corundum 1,000
10 diamond 140,000

Hemimorphic - Half formed crystals in which the faces that grow on one end are different in angle and position from the faces to be found on the other end. 

Idiochromatic - Minerals in which the color is due to an essential constituent. 

Imitation Stones - Substances used to look like a genuine. ie: glass, plastic, etc. 

Inclusions - Solid, gaseous, or liquid material of various types incorporated in a crystal during its formation and growth. Substances within a mineral, example, other minerals, gas bubbles, liquids, or other foreign objects.

Intaglio - (Italian) name for engraving with a negative image, as used for seals

Intarsia - Mosaic inlay work using colored stones. (and wood) 

Labradorescence - Patchy or diffuse iridescence due to the interference of light by reflection from parallel inclusions. Example Labradorite. 

Luster - how the mineral reflects light. It is described as metallic, vitreous (glassy), waxy, greasy, silky, etc.

Malleable - Can be flattened by pounding, as in metals. 

Mammillary - rounded mineral surface 

Meteorite - Solid object that comes from outer space and falls to earth. 

Moh's Scale - The relative scale of the hardness of minerals, from 1 to 10, the order of hardness with no significance to quantitative relationship. (See hardness)

Nicol Prism - Prism for producing polarized light, having split the light into two rays. 

Nodule - A lump or node. 

Opaque - Not transparent or translucent. (You can't see through it) 

Optic Axis - The direction of single refraction in a double refractive mineral. 

Optic sign - If the lower index of the refractive reading does not vary when the gemstone is rotated, it is uniaxial and + optic sign, if the higher index is steady and the lower index varies the optical sign is -. 

Ore - A mineral occurring in sufficient amounts to permit its recovery at a profit. 

Organic gemstones - This group includes materials that, even though they are of organic origin, have preserved or acquired a certain stone character. They are an important part of the trade, especially with respect to amber or pearl. Many are made up of minerals; pearls are mostly aragonite, bone is largely apatite.

Orient - Characteristic sheen and iridescence, as displayed in a good pearl. 

Paste - Lead glass imitation stone. 

Pegmatites - A very coarse platonic rock, generally granitic in composition. Usually forming dikes that cut granite or the gneisses and schists that border granite masses. They are coarse because the liquid residue at the time of their crystallization contained a high percentage of water and other volatile elements that did not go into the makeup of the common minerals of granite, and were concentrated in the residue. 

Phosphorescence - fluorescence that continues for a while (however short) after the ultraviolet light source is turned off.

Piezoelectric - A substance that becomes electrically charged by pressure. 

Pleochroism - a phenomenon of certain minerals whereby the same stone will exhibit different colors when viewed from different angles under the same light source. Dichroism indicates two colors will be seen, trichroism indicates three colors. The effect is the result of different light absorption patterns along different axes of the crystal.

Polarized Light - Light which vibrates in one direction or plane. 

Potch - native material in or around a gemstone, a dead spot or area. ie: ironstone surrounding opal. 

Quartz Species - Moh's hardness: 7
Chemical composition: SiO2, silicon dioxide
Macro crystalline quartz (crystals recognizable with the naked eye) includes stones gemologists classify as varieties of the quartz species: amethyst, aventurine, rock crystal, blue quartz, citrine, hawk's eye, prasiolite, quartz cat's eye, smoky quartz, rose quartz, and tigereye.

Radioactive - Emitting alpha, beta, or gamma rays. 

Reconstructed Stones - Old method of fusing Stones made from chips. Used now to describe amber re-melted and fused from otherwise unusable pieces. 

Reflection - The throwing off or back, light from the surface. 

Refraction - The bending of light passing through one medium to another. The changing of direction. 

Refractive Index - the measure of how much light is bent as it passes through the material. Technically, it's the ratio of light ray angles entering, passing through and exiting the mineral. Values for natural gem materials range from about 1.4 to 3.2. Generally speaking, the higher the value the greater the potential sparkle in a stone (if cut well).

Refractometer - Instrument used to determine the refractive index of various substances. 

Rock - Stony matter. Any mass of mineral matter forming an essential part of the earth's crust. 

Sclerometer - Device for the quantitative measuring of hardness. 

Sedimentary Rocks - formed originally of sediment, including shale and sandstone, composed of fragments of other rocks deposited after transportation from their sources, and including those formed by precipitation, as gypsum, or by calcareous secretions of animals as in certain limestones. 

Sheen - The iridescence of light reflected from the surface of a stone. ie: moonstone 

Specific Gravity - The weight of a substance compared to the weight of an equal volume of pure water at 4 degrees C. 

Spectroscope - Instrument which resolves light into its component wavelengths by refraction through prisms or diffraction grating. 

Stalactites, stalagmite - the formation of a cone-like structure that grows from the deposits of carbonate of lime build up. 

Star - The effect of proper cutting of a gemstone as a cabochon when the gemstone contains long rod like crystals or fibers in parallel or cavities where these have formally been. The reflection, (using a single light source), from the surface produces lines in even numbers, i.e.: 2,4,6,8,12, with a common center. 

Streak - the color of the powder from a mineral made when the mineral is dragged across a piece of porcelain. The trail, or streak, of powder can be a definitive color in some cases (e.g. hematite's streak is red).

Synthetic stones - Man made stones that have approx. the same composition and or crystal structure of the natural crystal that they represent. 

Transparency - The degree to which light passes through a substance. 

Twin Crystals - Two or more crystals which have grown together in a symmetrical fashion. 

Uniaxial - Minerals having single refraction, one optic axis. 

Vectorial properties - properties of a mineral that vary with direction. 

Vein - A more or less upright sheet deposit of minerals, cutting other rocks and formed from solutions rather than from a molten magma as in a dike. 

Vicarious elements - those that take each other's place in trace amounts in a minerals crystal structure. 

Vitreous - Glasslike in appearance or texture. 

Vug - An open cavity in rocks, often lined with crystals. 

Xenolith - rock enclosed in magma.