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Glossary of Selected Important Terms1

Glossary of Selected Important Terms


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.

Alluvium...Continental sediments due to transport and deposition of gravel, sand, and clay by running water, rivers, and streams. (See alluvial)

Alpha rays...Helium atoms with double positive charge.

Alpine cavities (vugs)...Hollows in silicate rock, they may be partially filled with mineral formations.
Alteration pseudomorph...One mineral has been replaced by another that is unrelated while preserving the original crystal form.

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.

Amygdaloidal...(amygdule) gas filled volcanic rock.

Amygdule...A rounded mass of mineral formed in a gas cavity.

Angstrom Unit...Unit and length of measurement of wavelengths of visible light and most xrays. (1 A.= .0000001 millimeter)

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... That rock in which the crystals are too small to be seen by the unaided eye.

Arid... dry or desert like.

Arsenates...Minerals in which AsO4 radical is an important constituent.

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.

Atom...The smallest part of a chemical element which remains unchanged during all chemical reactions. Atomic Weight...Weight of an atom compared with an atom of oxygen (16.00).

Batholith... A huge body of plutonic rock that has been intruded deep into the earth's crust and latter exposed by erosion.

Bean (pisolitic) iron ore... Globular aggregates of limonite that occur in karst cavities as weathering formations.

Beta rays...Electron rays
Bezel... A rim of metal surrounding a gemstone securing it.

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

Bipyramid (dipyramid)...Crystals that form symmetrically about a plane dividing it into two pyramids.

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.

Boule...The form and shape of a synthetic stone when created by the inverted blowpipe of a Verneuil furnace, somewhat carrot shaped.

Breccia...An aggregate of angular fragments of stone or mineral cemented together as in calcite and chalcedony.

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... A carved shell, sometimes cut from onyx or other mineral containing bands of different colors, To cut in relief, the opposite of intaglio.

Carat...Unit of weight used to weigh gemstones, equal to 200 milligrams, or .200 grams. 1 gr. = 5 ct. 100 points = 1 ct. metric system.

Cataclastic rock... A metamorphic rock produced by the crushing and grinding of preexisting rocks, which are still visible as crushed fragments.

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.

Chemical Element...Matter composed of atoms of only one chemical type which cannot be decomposed into simpler substances by chemical methods.

Clastic rock... Sedimentary rock made up of fragments of preexisting rocks and transported into the place of deposition.
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.

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