Welcome to the Mineral and precious stone glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z  All terms

R

Radiating

Aggregate composed of tiny, slender crystals compacted together radiating from a central point. The radiation can be flat or three dimensional. If three dimensional, this aggregate commonly occurs with circular, ball-like masses, and is known as spherulitic.

Radiation

The emission of alpha, beta, or gamma rays.

Radical

Compounds that act as a single atom when combining with other elements to form minerals. Radicals contain one or more unpaired electrons.

Radioactive

1) Term given to describe substances that contain atoms whose nuclei are are unstable, which causes the substance to slowly decay and emit radiation. Radioactive minerals usually take an extremely long time to decay if kept in proper settings.
2) (verb) - Emitting alpha, beta, and gamma rays.

Radioactivity

The effect of radioactive material, emitting alpha, beta, and gamma rays. Also used to describe in present tense the nature of radioactive material.

Rare earth

May either refer to the rare earth elements, or oxides of the rare earth elements (i.e. One or more of the rare earth elements combined with oxygen).

Reflection

The event occurring when light hits a material and bounces off of it. There are different degrees of reflection, the strongest being light rays bouncing off a smooth, flat, polished surface, such as a mirror, where they bounce off and form a reversed image on the surface. Polished Hematite is a great example describing a reflective mineral, where in some cases it produces a mirror-like reflection.

Reflective

Giving off or having to do with reflection.

Refract

Bend.

Refraction

The bending of white light upon entering a new dimension, such as from air to water or from air to a crystal, and splitting the white light into the colors of the spectrum.

Refractive index

The amount of refraction that takes place in a particular substance, which is a direct connection to the speed of light in that substance. The higher the refractive index, the greater the amount of dispersion, which increase the brilliance of a material. The refractive indices of gems are measured with a refractometer. Gems can be identified by measuring their refractive indices.

Refractometer

Instrument that measures the refraction of light when it enters through a different dimension, such as from air to water or from air into a crystal.

Reniform

Aggregate describing smooth, rounded, kidney-like agglomerations. Rounded agglomerations of reniform aggregates are larger than botryoidal agglomerations but smaller than mammilary agglomerations.

Repeated twinning

Form of twinning where two or more crystals form a repeated pattern. Examples: cassiterite twin, rutile twin, plagioclase twin, and fishtail twin.

Replace

Having some or all the atoms being exchanged for atoms of a similar element. See replacement.

Replacement

The process of one mineral taking the place of another mineral or material, with one or two atoms per molecule in the structure being exchanged with a different one with similar characteristics, thus creating a new mineral that retains the shape of the first mineral. (See Pseudomorph for more information.) Also refers to one element of a mineral to partially or fully substitute itself to another element.

Resinous (luster)

Luster describing yellow, dark orange, or brown minerals with slightly high refractive indices -- honey like, but not necessarily the same color.

Resistance

The unyielding of material to destructive acids and tough environments.

Reticulated

Aggregate composed of long crystals in a netting-like form, where all the crystals crisscross each other. (i.e. Crocoite)

Rhombohedral cleavage

Type of cleavage exhibited on minerals that crystallize in the hexagonal system and as rhombohedrons, in which small rhombohedrons cleave off of the existing rhombohedron. Example = Calcite.

Rhombohedron

Six sided polyhedron with each side shaped as a rhombus.
A mixture of a rhombohedron and scalenohedron also exists. It is sometimes called a rhomb-scalenohedron.

Rift

Crack in a rock. On a large scale it refers to the separation of plates on fault areas, forming a depression in the earth.

Rock

An indefinite mixture of naturally occurring substances, mainly minerals. Its composition may vary in containment of minerals and organic substances, and are never exact.

Rock crystal

Transparent, colorless, crystal of Quartz.

Rock shop

Commercial store where minerals and rough gemstones can be purchased.

Rock show

Periodical show in a large, usually indoor arena where mineral and gem dealers come to sell their stock at wholesale prices.

Rosette

Mineral with concentric aggregates resembling rose flowers.

Rough

Without any crystal faces. In regard to gemstones it refers to unfaceted material.

Rust

Powdery or crusty, brownish-yellow coating of iron oxides commonly see on iron materials after they come in contact with water.

Rusting

Developing a coat of rust on the surface, or containing a rust coating from a previous rusting.

Rutilated

Containing Rutile impurities.

Rutile twin

Two or more crystals that twin in a repeated pattern ("repeated twinning") as depicted in the figure below. Named after the mineral Rutile, which most frequently exhibits this form of twinning. If there are six repeated crystals (i.e. another three crystals are added to the figure below in the same pattern), the agglomerate forms a circular structure, known as a sixling.