(Study Material) Geology Study Material

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Alteration pseudomorph : One mineral has been replaced by another that is unrelated while preserving the original crystal form.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.

Conglomerate ( as in geology) .. Conglomerates, as well as sedimentary breccias, are coarse-grained SEDIMENTARY ROCKS formed by the consolidation and hardening of, respectively, rounded and angular gravel deposited in oceans. More than 30 percent of the large particles of these rocks exceed 2 mm (0.08 in) in diameter. The particles may be pebbles, cobbles, or boulders, or mixtures of these sizes. Both conglomerates and sedimentary breccias may be named and classified by the proportion of gravel - sized particles; the type of matrix,and the types of gravel-sized particles. The proportion of gravel is a function of the highest current speed at the time of deposition and the availability of particles of such coarse size. A sample that is more than 80 percent pebbles, cobbles, or boulders is called a conglomerate proper, whereas one that is 30 to 80 percent is an arenaceous (sandy) conglomerate or an argillaceous (shaley) conglomerate. The matrix between the layers of coarse particles may also be calcareous (that is, containing calcium carbonate) or sideritic (containing ferrous carbonate). On the basis of the variety of pebbles, cobbles, and boulders in conglomerates, they can be classified as oligomictic, consisting of a single kind of rock (such as one of various varieties of chert and quartzite or other rock), or polymictic, containing many kinds of rock.

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

Contact metamorphism : The change of rock due to the effect of high temperatures during contact with a lava flow, magma sloping, or igneous intrusion.

Critical Angle : The angle at which a ray of light passes from one medium to another, as a gemstone and air. Cryptocrystalline :  : 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.

Decrepitation :  The explosive shattering of mineral grains on heating.

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.

Detrital : Occurrence of minerals in gravels that came from a mineral deposit. (placer)

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

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

Dike :  In the forming of rocks, when intruding sedimentary rocks in a vertical or nearly vertical position.

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.

Doublets : A common method of building up sufficient thickness to permit a gem to be used in a setting. A non gem mineral is cemented to the top or bottom of the gem material. (See Opals.)

Dripstone : stalagmites or stalactites

Druse : A crystal coated surface of rock.

Doctile : Able to be drawn into a wire.

Endogenous : Generated deep in the earth by volcanism or earthquakes.

Enhydro : A chalcedony or carnelian geode having the center cavity filled with water.

Epithermal vein :  Formed at shallow depths from ascending hot solutions.

Epizone : In regional metamorphism, the depth level nearest the surface to approx. 3.75 to 4.25 miles.

Extinction : Diminution in the intensity of radiation due to absorption by or scattering in the medium; also the stopping of incident X-rays by the outer layers of atoms in a crystal.

Extraordinary Ray : Crystals and minerals belonging to the hexagonal tetragonal systems in which the ray or refractive index varies according to its direction through the crystal.
Extrusive rock : Igneous rock that solidifies on the surface of the earth.

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 : Temporary emission of radiations of different wavelength (color) by a substance struck by light waves usually long or short wave ultraviolet light.

Fluvial : Deposits from rivers.

Foliated : Made up of thin leaves, like mica.

Formulas : For weight estimation based on measurements in millimeters to 1/10. assuming well cut stones with little or no bulge factor. Some formulas for various gem cuts.

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.

Fumaroles : Spots in or near active volcanos where gases are expelled.

Gabgue : The minerals of no value in veins with ore minerals.

Gamma rays : Short wave electromagnetic waves.

Gangue : Minerals of no value associated in veins with ore minerals.

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 :  acrystal direction along which the atoms can slip a defined distance without destroying the coherence of the crystal.

Grain : (Troy system)..480 grains to the oz.

Habit : Characteristic crystal form.

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

Hardness : The resistance by a substance to actions which tend to modify its surface by scratching, abrasion, penetration.

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.

Hydrothermal : Hot water or solution sometimes superheated

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

Igneous rock.. Rock formed by the solidification of magma.

Inclusions : Substances within a mineral, example, other minerals, gas bubbles, liquids, or other foreign objects.

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.

Intaglio : Incised carving; a sunken design, ie: cameo

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

Ion : Electrically charged atom, radical or molecule.

Isomorphous  : Minerals in which two or more elements can replace each other to any extent without notably changing the appearance of the crystal.

Isotropic : Materials in which the optical character is the same in all directions.

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

Light : Radiant energy which gives the sensation of sight. Velocity is 186,285 miles per second.

Litmus paper :  colored paper used to show whether a solution is acid or alkaline.

Lopolith.. A large, lenticular, sunken mass of igneous rock whose surfaces are concordant with the enclosing rocks.

Luster : A reflective property of mineral surfaces.

Magma : Molten silica containing volatile substances in solution, present beneath the surface in certain areas of the earth's crust.

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

Mammillary :  rounded mineral surface

Massive : Minerals not bounded by crystal faces.

Metamorphism : (geological)..Changes in rocks brought about by heat and pressure acting in the rocks just below the surface.

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

Miaroles : Small cavities in granitic rocks created by volatile components.

Miller indices : Group of three digits used to designate crystal faces.

Mohs Scale : The relative scale of the hardness of minerals, from 1 to 10, the order of hardness with no significance to quantitative relationship.

Molecule : Two or more atoms in close relationship, The smallest quantity of an element or compound which is capable of independent existence.

Monochromatic Light : Light of one wavelength only.

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 : compounds of carbon, ie: coal, jet

Orient : Characteristic sheen and iridescence, as displayed in a good pearl. v Orientation :  Applied to crystals, this means visualizing the disposition of the principal directions within the crystal.

Orogenesis : A complex phenomena that leads to the formation of mountain ranges.

Outcrop : Where the bed rock is exposed.

Paleontology :  a division of geology that concerns itself with prehistoric life.

Paragenesis..The sequence in time in which a mineral crystallizes with respect to the other minerals.

Paramorph : A chemically identical crystal compared with the original, but the atoms have been rearranged so that they no longer conform to the original outline.

Parting : A smooth fracture that looks like a cleavage plane but takes place only on certain planes in the crystal, not between any set of atoms, like true cleavage.

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.

Petrography.. The branch of geology that deals with the description and classification of rocks.

Phaneritic rock.. An igneous rock in which all of the essential minerals can be seen by the unaided eye.

Phenocryst : Crystal that is set in a finer grained ground mass.

Phosphorescence : Luminescence emanating from substances that have been irradiated with ultraviolet light or X-rays, but persisting after the source of stimulation has stopped.

Piezoelectric : A substance that becomes electrically charged by pressure.

Pipe : In geology, the tubular ascending structures in volcanic rock.

Placer : Concentrations of heavy or resistant minerals that have been transported to rivers or other water areas.

Plastics : Synthetic resin, hard, usually molded.

Pleochroism : Dichroism and similar effects shown by some biaxial minerals in which three distinct colors or shades can be seen by using a dichroscope. (two at a time)

Plutonic rock : rock that has solidified deep underground.

Pneumatolysis : The action of gases of a magma on the surrounding rocks and on the solidifying molten mass itself. Point : (diamonds).. 1/100 of a carat, .01 ct.

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

Polaroid : Class or plastic which passes light on one plane.

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

Radioactive : Emitting alpha, beta, or gamma rays.

Reconstructed Stones : Old method of fusing Stones made from chips. Used now to describe amber remelted 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 measurement of the amount of change in direction of light passing through one medium to another. The ratio of the velocity of the light in air to the velocity of light in the medium being measured.

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

Rock : Stoney 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.

Selvage : The area bordering a vein.

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

Silk : Fine rod like crystals embedded in a crystal

Skarn : Contact rock of igneous silicate masses with limestone.

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.

Spectrum : The band of light showing the succession of rainbow colors corresponding to the different wavelengths. The VISIBLE spectrum is the small portion in the vast spectrum of electromagnetic waves, which extend from very long waves, (now used by submarines for underwater communication) to very short waves (gamma) emitted by radioactive elements.

Stalactites : dripstone, stalagmite..the formation of a conelike 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, ie: 2,4,6,8,12, with a common center.

Step cut : (trap cut)..Cut with edges are parallel to those the rectangular table.

Synthetic stones : Man made stones which have approx. the same composition and or crystal structure of the natural crystal which they represent. (See synthetic diamonds.)

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

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

Ultraviolet Light : Wavelengths of light shorter than visible violet light.

Ultrabasic rock.. Any plutonic igneous rock with very low silica content.

Uniaxial : Minerals having single refraction, one optic axis.

Vacuolar texture .. texture typical of igneous rocks rich in cavities.

Vectorial properties .. properties of a mineral which 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 mineral,s crystal structure. Vitreous.. Glasslike in appearance or texture.

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

Xenolith .. rock enclosed in magma.

Xrays : Electromagnetic radiations of a shorter wavelength than visible light. Usually less than one Angstrom.

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