P

Paleontology

The science and study of previous life forms, primarily in the form of fossils.

Paleontologist

Individual practicing in the subject of paleontology.

Paragenesis

The crystallization mode of one mineral in retrospect to other minerals. Paragenesis is applied to determine the conditions and qualifying factors necessary for a mineral to form.

Paramagnetic

Weakly attracted to magnetic fields.

Paramagnetism

Magnetic property in certain iron bearing minerals that cause them to be weakly attracted to magnetic fields.

Paramorph

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 (CaCO3 - Crystallizes in orthorhombic system) that becomes unstable and transforms into Calcite (CaCO3 - Crystallizes in trigonal system), but retains the original orthorhombic shape.

Parting

The tendency of certain minerals to split along stressed areas or along twinned crystals. (See Parting in mineral properties for more information.)

Pearl

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.

Pegmatite

Area of igneous rock where the individual grains are very large, meaning that large crystals occur there.

Pendant

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

Penetration twinning

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

Penetration

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

Percussion figure

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

Petrification

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

Petrified

Having been replaced by silica after once being organic. See also petrification. Petrified Wood

Petrified wood

Wood that is petrified (i.e. replaced by silica).

Petrographer

Individual practicing in the subject of petrography.

Petrography

Type of geology that deals with the classification of rocks, which is based on the material they contain.

Petrographic microscope

Special microscope used to determine the makeup of rocks in order to classify them. See petrography.

Petrology

Type of geology that deals with the formation, composition, and source of rocks.

Petrologist

Individual practicing in the subject of petrology.

pH

Measurement used to survey the amount of acid or alkaline present in a material. pH is measured on a scale of 0 - 14. Neutral is 7; numbers below 7 are acid and above are alkaline. The lower the number, the stronger its acidity, and the greater the number, the stronger the alkalinity. pH stands for "potential of Hydrogen" (for hydrogen causes a material to be acidic).

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.

Phenocryst

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

Phosphates

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. (In this guide, "true phosphates" refers only to minerals with the phosphate radical, excluding the arsenates and vanadates which contain the arsenate and vanadate radicals.)

Phosphorescence

The ability of some fluorescent minerals to keep on glowing for several seconds after the ultraviolet source has been removed. (The phosphoresce may cause the mineral to glow for only several seconds, but the phosphorescent effect really lasts much longer -- several seconds is only what the eye can perceive.

Phyllosilicates

Group of silicate minerals that have each set of tetrahedrons surrounded by three oxygen atoms, forming a sheet like structure. For more information, see "All About the Silicates Group".

Physicist

Individual who specializes in physics.

Piezoelectric

Substance that generates an electrical charge when under stress.

Piezoelectricity

Electricity generated when a piezoelectric substance is put under stress.

Pinicoid

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

Pinicoidal

By way of the pinicoid. When in reference to a crystal type, it refers to an elongated crystal.

Pinicoidal cleavage

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.

Pisolite

Any mineral that is pisolitic.

Pisolitic

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

Pit

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

Pitchy

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

Area in a stream or river where heavy material settles after being carried downstream in the current of the river or stream.

Plagioclase twin

Two or more crystals that twin in a repeated pattern ("repeated twinning") as depicted in the figure below. Named after the mineral Plagioclase, which most frequently exhibits this form of twinning.

Plane

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 (i.e. if rotated 180° from that line will yield the same shape).

Plate

Small, flat, flaky crystal. Also the definition of the section of rock present by fault areas.

Platy

Small, flat, and flaky.

Playa

A flat plain in a low point in a desert where rain collects after a storm.

Pleochroic

Displaying pleochroism.

Pleochroism

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.

Plug

The solidified "cover" over the opening of a dormant volcano.

Plumose

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

Plutonic rock

Igneous rock that solidified in the crust of the earth, and individual crystal grains can be seen.

Pocket

Cavity in igneous rock in which crystals are usually found.

Point

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

Polished

Either tumbled, faceted, or coated to enhance luster.

Polychromatic

Minerals that are polychromatic have many different color variances.

Polyhedron

A three dimensional figure composed of specific shapes.

Polymorph

A mineral with the chemical equivalence (i.e. containing the same amount of the same elements) of another mineral but different atomical arrangement. Two examples are:
1) Diamond and Graphite
2) Rutile, Brookite, and Anatase

Polymorphism

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

Polymorphous

Being polymorphs to one another.

Polysynthetic twinning

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

Polysynthetic twins

Twinned crystals that are twinned through polysynthetic twinning.

Porcelaneous

Containing rounded, tiny holes throughout.

Porous

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

Porphyritic

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

Porphry

Igneous rock containing large, noticeable crystals, usually feldspars.

Precious stone

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

Precipitating

Going through precipitation, meaning releasing dissolved mineral.

Precipitation

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

Primary

Forming as new, not as an alteration product. (See "primary mineral" for more information)

Primary mineral

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

Prism

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

Prismatic

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

Prismatic cleavage

Type of cleavage exhibited on some prismatic minerals in which the mineral cleaves by breaking off thin, vertical, prismatic crystals off of the original prism. Example = Acmite.

Prospect

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

Prospecting

Searching for a mineral deposit or a mineral in a deposit.

Prospector

Individual who prospects minerals.

Proton

Positively charged particle of an atom that resides in its nucleus.

Pseudo

Assuming a false shape.

Pseudocube

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.)

Pseudocubic

Assuming the shape of a pseudocube.

Pseudohexagon

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.

Pseudomorph

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.

Pseudomorphed

A mineral that has gone through pseudomorphism.

Pseudomorphism

The act of one mineral chemically replacing another. See also pseudomorph.

Pure

Free of impurities.

Pyncnometer

Type of instrument that measures specific gravity.

Pyramidal

Shaped as a pyramid (i.e. with a pointed top).

Pyritohedral

Shaped as a pyritohedron

Pyritohedron

Twelve sided polyhedron; all sides are equidimensional and pentagonal. Minerals shaped as pyritohedrons belong to the isometric system. Plural = Pyritohedra or Pyritohedrons

Pyroelectric

Substance that generates an electrical charge during a temperature change.

Pyroelectricity

Electricity generated when a pyroelectric substance is put under temperatures in which it will generate electricity.

Pyrometallurgy

The separation of metals from ore or from alloys through a process dependent on heat, or the forming of alloys and purification of metals through a process dependent on heat. See also metallurgy.

Pyroxene

Individual member of the pyroxene group.

Pyroxene group

Group of minerals that contain iron, magnesium, silicon, and oxygen. They may also contain calcium, sodium, and lithium. The pyroxenes have prismatic cleavage, and one can note the angle of 87° and 93° (almost perpendicular), which distinguishes it from amphiboles which are at 56° and 124°. The pyroxenes are very similar to the amphiboles, and the minerals belonging to each group many times cannot be distinguished by ordinary methods. Generally, though, the pyroxenes occur in shorter, stubbier crystals than the amphiboles. The amphiboles contain the same elements that the pyroxenes do, except they have hydroxyl (OH) in their structure, which alters both its physical and chemical properties. The pyroxene group is in the inosilicate subdivision of the silicates group.