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Inland Lapidary - Gem glossary
Category: General technical and industrial > Gems and Geology
Date & country: 27/10/2013, USA
Words: 1119

A pseudomorph involving two minerals with an identical composition but different crystal structures. The original mineral forms, but conditions then cause it to be unstable, so it transforms into the other mineral with the same chemical structure while retaining the original crystal shape. An example of this is Aragonite that becomes unstable ...

parent isotope
A radioactive isotope that changes into a different isotope when its nucleus decays. See also daughter isotope.

parent material
The source from which a given soil is chiefly derived, generally consisting of bedrock or sediment.

partial melting
The incomplete melting of a rock composed of minerals with differing melting points. When partial melting occurs, the minerals with higher melting points remain solid while the minerals whose melting points have been reached turn to magma.

The tendency of certain minerals to split along stressed areas or along twinned crystals.

passive continental margin
A border that lies between continental and oceanic lithosphere, but is not a plate margin. It is marked by lack of seismic and volcanic activity.

Smooth, round, shiny, organic object composed mainly of calcium carbonate found in the shells of some mollusks. Not to be confused with mother-of-pearl.

pearly luster
Exhibiting a luster similar to the inside of a mollusk shell or shirt button. Many mica's exhibit a pearly luster, and some minerals with a pearly luster have an iridescent hue. Some minerals may exhibit a pearly luster on cleavage cracks parallel and below the reflecting surface of a mineral.

A soft brown mass of compressed, partially decomposed vegetation that forms in a water-saturated environment and has a carbon content of 50%. Dried peat can be burned as fuel.

A broad surface at the base of a receding mountain. The pediment develops when running water erodes most of the mass of the mountain.

A coarse-grained igneous rock with exceptionally large crystals, formed from a magma that contains a high proportion of water.

Ornament or piece of jewelry that hangs down, such as from a necklace or earrings.

The formation of a crystal penetrating through rock or another crystal.

penetration twinning
Form of twinning where two or more crystals are intergrown. Examples: fluorite twin, carlsbad twin, staurolite twin, and swallowtail twin.

Pentagonal dodecahedron
Synonym of pyritohedron.

perched water table
A saturated area that lies within a zone of aeration.

percussion figure
Six rayed, star-like flaw that forms when some micaceous minerals are put under pressure.

An igneous rock composed primarily of the iron-magnesium silicate olivine and having a silica content of less than 40%.

Permanently frozen regolith, ranging in thickness from 30 centimeters to over 1000 meters.

The capability of a given substance to allow the passage of a fluid. Permeability depends on the size of and the degree of connection among a substance's pores.

petrification (petrifaction)
Process in which organic substances, such as wood and shells, are replaced by silica.

petrified wood
Wood that is petrified, i.e. replaced by silica.

Type of geology that deals with the classification of rocks, which is based on the material they contain. Person who studies in this discipline is a petrographer.

Any of a group of naturally occurring substances made up of hydrocarbons. These substances may be gaseous, liquid, or semi-solid.

Type of geology that deals with the formation, composition, and source of rocks. Person who studies in this dicipline is a petrologist.

phantom growth
An interesting phenomenon exhibited when a crystal grows, than a new growth grows over the old crystal in the same direction, leaving an inscription of the previous growth on the crystal. Additional growth may be present, leading to the possibility of more than one phantom in a crystal.

Large crystal surrounded by much smaller crystals in porphyritic igneous rock.

phosphates group
Group of minerals that are compounds of one or more metallic elements associated with the phosphate radical (PO4). The phosphates are classified together with the arsenates and vanadates. Most phosphates are heavy, and none are hard. They are usually brittle and occur in small crystals or compact aggregates.

Emission of visible light by a substance, such as a mineral, that is exposed to ultraviolet light and absorbs radiation from it. The light appears in the form of glowing, distinctive colors. The emission continues after the exposure to ultraviolet light ends.

A foliated metamorphic rock that develops from slate and is marked by a silky sheen and medium grain size.

Group of silicate minerals that have each set of tetrahedrons surrounded by three oxygen atoms, forming a sheet like structure.

The study and science of energy and motion of matter. The person who studies this discipline is a physicist.

Substance that generates an electrical charge when under stress.

Electricity generated when a piezoelectric substance is put under stress.

The third dimension of a three dimensional figure, usually representing its width.

When in reference to a crystal type, it refers to an elongated crystal.

pinicoidal clevagge
Type of cleavage exhibited on some prismatic and tabular minerals where they cleave on the pinicoidal plane, which is the third dimension aside from the basal and prismatic sides, which they may also cleave on.

Tube-like, cylindrical body of igneous rock.

Aggregate composed of small, spherical particles, larger in size and commonly more distorted than oolitic minerals.

Type of mine where a large hole is dug in the ground to extract the valuable material.

pitchy (pitchlike) luster
Luster of a mineral that appears similar to tar. Minerals with a pitchy luster are radioactive and have gone through the process of metamiction.

placer deposit
A deposit of heavy or durable minerals, such as gold or diamonds, typically found where the flow of water abruptly slows.

Imaginary line connecting two points on a surface.

plane of symmetry
Imaginary lines traced on polyhedrons such as cubes and octahedrons depicting a point on the polyhedron that exhibits symmetry. For example, if rotated 180� from that line will yield the same shape.

Synonym of asteroid

plastic deformation
A permanent strain that entails no rupture.

1. Small, flat, flaky crystal. 2. The definition of the section of rock present by fault areas.

plate tectonics
The theory that the Earth's lithosphere consists of large, rigid plates that move horizontally in response to the flow of the asthenosphere beneath them, and that interactions among the plates at their borders cause most major geologic activity, including the creation of oceans, continents, mountains, volcanoes, and earthquakes.

Small, flat, and flaky.

A dry lake basin found in a desert.

Pleistocene Epoch
The first epoch of the Quaternary Period, beginning 2 to 3 million years ago and ending approximately 10,000 years ago. See also Holocene Epoch.

The effect present in a mineral exhibiting two or more separate colors when viewed at different angles. Pleochroism and dichroism are synonymous, except dichroism refers only to two colors, but pleochroism can be more than two. Minerals displaying this characteristic are said to be pleochroic.

Featherlike aggregate in which many small crystals protrude out of a long, slender one.

An intrusive rock, as distinguished from the preexisting country rock that surrounds it.

plutonic rock
An intrusive rock formed inside the Earth crust, and individual crystal grains can be seen.

pluvial lake
A lake that formed from rainwater falling into a landlocked basin during a glacial period marked by greater precipitation than is found in the region in prior or subsequent periods.

Cavity in igneous rock in which crystals are usually found.

Unit of measurement given to small, precious gemstones. One point is equivalent to 1/100th of a carat. The abbreviation for point is Pt.

point bar
A low ridge of sediment that forms along the inner bank of a meandering stream.

Either tumbled, faceted, or coated to enhance luster.

Minerals that are polychromatic have many different color variances.

A three dimensional figure composed of specific shapes.

A mineral that is identical to another mineral in chemical composition but differs from it in crystal structure. Eganpmes: Diamond and graphite or Rutile, Brookite, and Anatase.

The tendency of minerals with the same chemical composition to form different crystal structures.

polysynthetic twinning
Form of twinning where the crystals intergrow in a repeated pattern.

polysynthetic twins
Twinned crystals that are twinned through polysynthetic twinning.

Containing rounded, tiny holes throughout.

The percentage of a soil, rock, or sediment's volume that is made up of pores.

Containing rounded, tiny holes throughout. Many porous minerals can be dyed.

Describing a rock that contains large, noticeable crystals, usually feldspars.

Igneous rock containing large, noticeable crystals, usually feldspars.

porphyry copper deposit
A crystallized rock, typically porphyritic, having hairline fractures that contain copper and other metals.

potassium-argon dating
A form of isotope dating that relies on the extremely long half-life of radioactive isotopes of potassium, which decay into isotopes of argon, to determine the age of rocks in which argon is present. Potassium-argon dating is used for rocks between 100,000 and 4 billion years old.

The combined influence of gravity and water pressure on groundwater flow at a given depth.

potentiometric surface
The level to which the water in an artesian aquifer would rise if unaffected by friction with the surrounding rocks and sediments.

precious stone
Gem or gemstone that is highly appealing and very costly, exhibiting a powerful luster, high hardness, and rarity.

The process in which dissolved mineral gets freed from water, forming a deposit.

Forming as new, not as an alteration product.

primary coast
A coast shaped primarily by nonmarine processes, such as glacial erosion or biological processes.

primary mineral
Mineral that forms by the combination of elements rather than by alteration of a mineral.

primary wave
A body wave that causes the compression of rocks when its energy acts upon them. When the P wave moves past a rock, the rock expands beyond its original volume, only to be compressed again by the next P wave. P waves are the fastest of all seismic waves. See also S wave.

principle of faunal succession
The scientific law stating that specific groups of animals have followed, or succeeded, one another in a definite sequence through Earth history.

principle of original horizontality
The scientific law stating that sediments settling out from bodies of water are deposited horizontally or nearly horizontally in layers that lie parallel or nearly parallel to the Earth's surface.

principle of superposition
The scientific law stating that in any unaltered sequence of rock strata, each stratum is younger than the one beneath it and older than the one above it, so that the youngest stratum will be at the top of the sequence and the oldest at the bottom.

principle of uniformitarianism
The scientific law stating that the geological processes taking place in the present operated similarly in the past and can therefore be used to explain past geologic events.

Crystal that is elongated in one direction; the other directions are about equal.

Crystal habit describing a crystal with four or more sides similar in length and width. Prismatic crystals are usually elongated in one direction.


Physical properties of a mineral or gem that have to do with optics, such as dispersion, absorption spectra, refractive index, asterism, and dichroism, just to name a few.

A characteristic that distinguishes one substance from another.

To search for a mineral deposit or mineral in a deposit.

A positively charged particle that is found in the nucleus of an atom and has a mass approximately 1836 times that of an electron.

Assuming a false shape.

Rhombohedron shaped crystal almost identical to a cube, but its angles slightly differ from a cube. (May also refer to any crystal that closely resembles any member of the isometric system but is slightly asymmetrical.)

Six sided crystal that assumes a hexagonal shape although it is not in the hexagonal system. The cause of pseudohexagonal crystals is orthorhombic crystals that intergrow in three individuals, forming six sided trillings.

One mineral that chemically replaces another mineral without changing the external form of the original mineral. There are three types of pseudomorphs: paramorphs, infiltration pseudomorphs, and incrustation pseudomorphs.

The act of one mineral chemically replacing another.

Abbreviation for point, 1/100 of a carat.

Free of impurities.

Type of instrument that measures specific gravity.