Glossary of Terms

In affiliation with GemologyOnline.com
From The Gemology Project

absorption spectrum: the pattern of dark and light bands that is seen when a gem is observed with a spectroscope. These bands result from the absorption of certain wavelengths of white light passing through the stone. The colors that are NOT absorbed determine the color of the gem.

acicular: crystals that have a "needle-like" form; such as rutile in quartz.

Adularescence: a billowing flow of whitish or bluish colors that seem to float along the surface; caused by the diffused reflection of light from parallel intergrowths of albite and orthoclase feldspar.
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allochromatic: a gemstone is allochromatic when it is colorless in its pure state. Subsequent color is derived from an impurity (usually a metallic oxide) that is not an essential part of the mineral's chemical composition. Beryl, quartz and corundum are all examples of allochromatic gemstones.

anisotropic: a term for crystals that are doubly refractive, which means that they will break light into 2 different rays, traveling at different speeds within the crystal.

asterism: star-like phenomenon caused by light reflecting from tiny fiber-like inclusions that are perpendicular to the crystal faces; the number of rays depends on the cut of the gem in concert with the orientation of the inclusions.
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biaxial: term used to describe crystals with two directions of single refraction. These are minerals crystallizing in the orthorhombic, monoclinic and triclinic crystal systems

birefringence: double refraction; the difference between the minimum and maximum refractive index of a gem (although strictly birefringence we should name that "maximum birefringence")

botryoidal: interlocking, rounded masses that sometimes look like grapes or bubbles resulting from radiating masses of fibrous crystals.
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bruting: The cutting of one diamond with another. It is only used in the production of round stones, in order to round out the girdle of the diamond on a lathe

cabochon: A type of gemstone cut where the back of the stone is flat (or slightly domed) and the top is formed as a smooth rounded dome.
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carat:a unit of weight: 1/5th of a gram. The name comes from the seed of the carob tree, which was used as a weight due to its seeds' remarkable uniformity.

chatoyancy: the cat’s-eye-like phenomenon caused by light reflecting from tiny fiber-like inclusions within a gem. The "eye" is seen at right angles to the direction of the inclusions. Stones must be cut en cabochon to see this effect.
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cleavage:

contact zone: the area where intruding magma or hot water contacts, alters and recrystallizes the pre-existing surrounding rock

color zoning
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cryptocrystalline: a term originally used to describe a mineral made up of "sub microscopic components". It is rarely used today since microscopes have become so sophisticated "sub microscopic components" no longer exist. Microcrystalline is a better description.

crystal: a solid possessing an orderly and defining arrangement of atomic structure, influencing its physical form and optical properties

crystal axes: imaginary reference lines used to determine the relative position of crystal faces

crystal systems: the seven main systems into which crystals are divided

crystallography: the study of crystals and their structure

cubic system: one of the crystal systems; composed of 6 square faces at 90° angles to each other. Also known as isometric. See Crystal Systems & Forms
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dichroism: differential selective absorption seen in some doubly refractive gemstones when viewed in different crytallographic directions

dichroscopes: instrument used by gemologists to test for pleochroism · London dichroscope · calcite dichroscope

double refractive: all crystals, except cubic, have the ability to split light into two rays when it enters. These 2 rays travel at different velocities. This is termed double refraction.

emission lines:

epigenetic inclusions: inclusions that form after the gemstone finished crystallization. If the conditions the crystal is in changes (the heat and/or pressure), material held in solid solution can be forced out and crystallized. Oriented rutile is an an example. Also, secondary cavities can form when fractures in stones are healed. In the process, characteristic patterns of many tiny crystals or negative crystals are formed.

facet: When used as a noun, a facet is a flat surface (one of many) cut into a piece of gem rough in order to maximize the reflection of light out of the stone. When used as the verb to facet, it means to cut a gemstone into a faceted shape, such as a round brilliant cut, emerald cut, cushion cut or similar.
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fluorescence: the emission of visible light by a gemstone when exposed to a light source whose light we normally cannot see. When the gemstone is exposed to ultraviolet light (UV), which falls outside the range of light that we can see, the UV light is absorbed by the gemstone. Due to processes inside the gemstone, it will lose energy. This loss of energy causes the UV light to change to a color in the visible light range (red, orange, yellow, green, blue, indigo or violet).

hexagonal system: one of the crystal systems; having three axes equal in length at 120° to each other, with the C or vertical axis at 90° to the other axes. See Crystal Systems & Forms.
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idiochromatic: A gemstone is idiochromatic when the element causing its color is an essential part its chemical composition. For example, iron, which is an intrinsic part of the chemical makeup of peridot, is the cause of its green color.

inert: having no change, movement, or reaction. In chemistry, the term inert is used to describe something that is not chemically active.

igneous rocks: rocks that formed at very high temperatures from siliceous (silica rich) melts.

imitations: materials used to mimic a gemstone without having the same composition as the stone it is imitating. Example: synthetic color-changing sapphire used to imitate alexandrite; cubic zirconia used to imitate diamond.

impurities: elements in the crystal structure that are foreign to the basic chemical composition of the gem.

inclusions: crystals, liquid- or gas-filled cavities that have been enclosed within a gem or mineral. Often peculiarly diagnostic in determining the identity of a gemstone. Example: bysollite inclusions in demantoid garnet.
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infrared light: Also known as heat, these wavelengths are beyond the visible red: between 790nm and 1,000,000nm on the electromagnetic spectrum. These radiations can produce a reaction with some gems and minerals. See luminescence and thermoluminescence.

interference figure: the figure seen when anisotropic gemstones are viewed in convergent polarized light. This figure may be diagnostic in determining the identity of a gemstone.

isometric system: another name for the cubic system.

isomorphous replacement: replacement of elements in a mineral's composition by other elements with the same valency. These elements do not substantially alter the crystal structure of the gem but can cause wide variations in the gem's optic and physical properties. The garnet group is an excellent example of isomorphic replacement.

isotropic: gems and minerals that are singly refractive. This means that light that enters the medium travels as one ray at one velocity in all directions. All cubic gems and amorphous substances are isotropic. See anisotropic.

karat: the measure used to describe the purity of gold. 24 karat is pure gold, 100% or 24 of 24 parts gold; 18 karat is 75% gold, or 18 parts gold and 6 parts alloy metals; 14 karat is 58% gold, or 14 parts gold with 10 parts alloy metals.

labradorescence: the phenomenon displayed by labradorite (caused by lamellar formation of lattice structure), which has an appearance of a predominantly bluish or greenish oil slick on water
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marble: a metamorphic rock created when a certain type of limestone is subjected to tremendous heat and pressure.

massive: used to describe crystals that have no apparent crystal form or are masses of smaller crystals.

metamorphic rock: igneous or sedimentary rocks that have been altered or recrystallized by extreme heat or pressure.

microcrystalline:

monoclinic system: one of the crystal systems; all axes sides meet at 90°, with all the axes being of different lengths. Two of the axes are prependicular, with the other situated at 90° to them. See Crystal Systems & Forms.
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opalescence: a reflection of a milky or pearly light from a gem's interior, sometimes used as a synonym for irridescence.

optic character: a stone's property of being either isotropic, uniaxial or biaxial; found by determining how the light travels down crystal axes; common instruments used to determine optic character are polariscope and conoscope

optical interference figure: see interference figure

optic sign: Uniaxial and biaxial gems can be further subdivided as being optically positive or negative. In uniaxial gems, if the refractive index value of the extraordinary ray is greater than the ordinary ray, the gem is positive and vice versa. In a biaxial gem, if the intermediate refractive index value is closer to the higher value, it is positive; if closer to the lower value, negative. Examples: Quartz is uniaxial positive; topaz is biaxial positive.

orange peel: a surface appearance resembling the outer skin of an orange. This is sometimes seen in plastic and glass similants and should be observed in reflected light.

orthorhombic system: one of the crystal systems; three axes all meet at 90° to each other, with all the axes being of different lengths. See Crystal Systems & Forms.
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pegmatite: an igneous rock, rich in quartz and feldspar with very large grains, indicating slow cooling

phantom crystal: also known as "ghost crystal", they occur in quartz when there is an interruption in the growth cycle. It appears like a faint crystal within a crystal

pleochroism: the appearance of more than one color (usually when viewed with a dichroscope) as a function of the crystallographic direction from which one is viewing a gem. It is caused by the selective absorption of the ordinary and extraordinary rays in uniaxial gems (dichroism) and, in biaxial gems, by the selective absorption in the 3 principal vibrational directions of the crystal (trichroism).

polarized light: light that is vibrating in one direction only. Doubly refractive gems polarize light into 2 rays which travel at right angles to each other.

protogenetic inclusions: inclusions that form BEFORE the gem. The gem crystal traps the inclusion as it grows. Burmese rubies from Mogok often exhibit protogenetic inclusions.

refractive index: The degree that light is bent when it enters a stone. This is measured with a refractometer. Most gems have refractive indices that range between 1.43 and 1.98. Diamond has a refractive index of 2.42, which means it bends light 2.42 times more than air!

rock: a geological unit made up of one or more minerals. The properties of rocks can vary widely depending on the varying percentages of minerals in their makeup

rough: a gemstone in its brute state, before undergoing cutting or carving
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schist: a metamorphic rock containing layers of different minerals that can be described as foliated or fissile.

sedimentary rock: rocks composed as the result of weathering and erosion of pre-existing rocks. Wind, water, and frost are able to wear away the highest mountains and the hardest rock masses, redepositing them in lakes, rivers and oceans, where they compact and form new rocks.

selective absorption: property of absorbing a particular selection of the wavelengths of white light as they pass through a stone. If all wavelengths of light pass through a stone evenly, it appears colorless. If the stone absorbs all wavelengths equally, it appears black. If it absorbs certain wavelengths and reflects all others, it will appear opaque and colored. If it absorbs certain wavelengths and transmits all others, it will appear transparent and colored.

singly refractive: means that a light ray will pass through a crystal as one ray, not split or polarized; isotropic stones (cubic system) are all singly refractive

spectroscopy, absorption: a technique (using a spectroscope) of measuring how much light of a particular wavelength (color) is absorbed by a gemstone. Color is often related to the presence of a particular element. Absorption spectroscopy is an inexpensive way to test for the presence of elements within a gem.
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spot reading method: finding the refractive index of a cabochon cut stone by finding where the light in the refractometer appears as half light/half dark on the reading scale
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strain/interference colors: evidence of internal strain that appears as rainbow-like colors (primarily reds and oranges) under a polariscope

syngenetic inclusions: inclusions that form simultaneously with the gemstone. The gem grows at varying rates, forming enclosed cavities that can be filled with solids, liquids or gases or combinations.

synthetics: Man-made gemstones having the same physical and chemical composition as the natural gemstones. Lab grown.

table: the flat top of a faceted stone
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tetragonal system: one of the crystal systems; three axes meeting at 90°, with the C axis being longer than the A and B axis (which are the same length). See Crystal Systems & Forms.
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trade name: the name given to a gemstone usually for marketing purposes, ie: tanzanite for ziosite, Tashmarine ™ for diopside, mandarine garnet for spessartite.

trichroic: anisotropic stones that may display three colors; a type of pleochroism

triclinic system: one of the crystal systems; all the axes are different lengths, with no angles meeting at 90°. See Crystal Systems & Forms.
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trigonal: one of the crystal systems that is considered by some gem references to be a subsystem of the hexagonal system. A trigonal prism has the effect of being a three sided prism. See Crystal Systems & Forms.
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twinning: when two or more crystals of the same species grow together during the formation period; can interlock or grow from the edge outward. Shows a "venetian blind" effect under magnification

ultraviolet light (UV): energy in wavelengths too short to be seen by the human eye, beyond the visible violet, measuring 100 to 380 nanometers. Some gemstones, when exposed to UV, emit colors very bright and different from their normal colors. SWUV light is harmful to the eyes so protective goggles should be worn.

uniaxial: the optic character of anisotropic minerals, meaning they have one direction of single refraction: Tetragonal, trigonal and hexagonal crystals are uniaxial.

white light: light consisting of all colors and wavelengths


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