Glossary of Terms
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 absorped 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.
allochromatic: a gemstone is allochromatic when it is colorless in it's pure state. Subsequent colour 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 phenomena caused by light traveling along tiny fiber-like inclusions that are perpendicular to the crystal faces; the number of rays depends on the crystal habit of the gem in concert with the nature of orientation of the inclusions.
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birefringence: double refraction; the difference between the minimum and maximum refractive index of a gem
botryoidal: interlocking, rounded masses that sometimes look like grapes or bubbles resuting from radiating masses of fibrous crystals
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
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 it's 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.
Show me Chatoyancy
contact zone: the area where intruding magma or hot water contacts, alters and recrystallizes the pre-existing surrounding rock
Show me Color zoning
cryptocrystalline: a term originally used to describe a mineral made up of "sub microscopic components". It is rarely used today as microscopes have become so sophisticated "sub microscopic components" no longer exist. Microcrystalline is a better description.
crystal: a solid possesing an orderly and defining arrangement of atomic structue, influencing its physical form and optical properties
crystal axes: imaginary reference lines used to determine the relative position of crytal faces
crystallography:the study of crystals and their structure
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.
epigenetic inclusions: inclusions that form after the gemstone finished crystallization. If the conditions the crystal is in, changes (heat and or pressure), material held in solid solution can be forced out and cyrstalize. Oriented rutile would be 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.
Fluorescence: Fluorescence is 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).
idiochromatic: a gemstone is idiochromatic when the element causing it's colour is an essential part it's chemical composition. For example, iron, which is an instrinsic part of the chemical makeup of Peridot is the cause of it's green colour.
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 temperaturs from siliceous (silica rich) melts.
imitations: materials used to mimic a gemstone without having the same composition as the stone it is imitating. ex: Synthetic color changing sapphire used to imitate Alexandrite or cubic zirconia used to imitate a diamond.
impurities: elements in the crystal structure that are foreign to the basic chemical composition of the gem.
inclusions: crytals, liquid or gas filled cavitites that have been enclosed within a gem or mineral. Often extremely diagnostic in determining the identity of a gemstone. Example: Bysollite inclusions in demantoid garnet.
infrared light: Also known as heat, these wavelengths are beyond the visible red: between 790Nm and 1,000,000Nm on the . 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 can be diagnostic in determining the identity of a gemstone.
isomorphous replacement: replacement ot elements in a mineral's composition by other elements with the same valency. These elements do not substantailly alter the crystal structure of the gem, but can cause wide variations in the gems' optic and physical properties. The Garnet Group is an excellent example of isomorphic replacement.
isotropic: gems and minerals that are singly refractive are termed isotropic. 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 anistropic.
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 an predominantly bluish or greenish oil slick on water Show me Labradorescence
marble: a metamorphic rock created when a certain type of limestone is subjected to tremendous heat and pressure.
massive: used to describe cystals 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.
opalescence: a reflection of a milky or pearly light from a gem's interior. Sometime 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 visa 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.
pegmatite dike: a place where magma, or sometimes a hot water solution caused by the magma, has pushed into a preexisting rock
phantom crystal: when two crystals grow in the same physical space
plane polarized light: light waves moving in one direction on one plane
pleochroism: property of showing more than one color or shade of a color
polarized: when waves move parallel in one direction; moving toward a pole or axis
protogenetic inclusions: inclusions that form BEFORE the gem. The gem crystal traps the inclusion as it grows. Burmese rubies often exhibit protogenic inclusions.
rhomboidal: lopsided, having unequal sides and no right angles
rock: a combination of several minerals
rough: a gemstone in it's brute state before undergoing cutting or carving
schist: dark rock that forms around a pegmatite intrusion
sedimentary rock: rock formed from igneous and metamorphic rock eroding and washing onto the sea floor
selective absorption: property of absorbing a particular selection of the white light spectrum
single refractive: means that a light ray will pass through a crystal as one ray; isotropic stones (cubic system)
spectrum: the band of colors that is emitted by a light source; measured in nanometers (nm)
Show me Spectrum
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
Show me Spot Reading
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 cavitites that can be filled with solids, liquids or gasses or combinations.
synthetics: Man made gemstones having the same composition as the natural gemstones. Lab grown.
table: the flat top of a faceted stone
tabular: grows as layers that are usually fairly easy to separate
trichroic: anisotropic stones that may display three colors are; type of pleocroism
twinning: two of the same species grow together during the formation period; can interlock or grow from the edge outward. Shows a "venitian blind" effect under magnification
ultraviolet light (UV): energy in wavelength too short to be seen by the human eye unless they are slowed down; some gemstones under UV display colors very bright and different from their normal colors; very harmful to the eyes so protective goggles should be worn
uniaxial: dichroic stones producing two colors
white light: everyday light we see with; a combination of color-related light rays that mix together to generate the white light used by our eyes