Glossary of Terms

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absorption spectrum: the spectrum pattern of colors, bands, and bars indicating what colors are trasmitted and what colors are being absorbed

acicular: means "needle like"; jumbled intergrowths of long, spiny crystals

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 another feldspar that has a slightly different refractive index from the main mass

anistropic: a stone that shows more than one color; double refractive; stones of all systems except cubic are anistropic

asterism: star-like phenomena caused by light traveling along tiny fiber-like inclusions that are perpendicular to the crystal faces; the number of stars depends upon the type of inclusion in the particular variety of stone

biaxial: trichroic stones that show two or three colors; all gemstones showing three colors are biaxial but not all biaxial gemstones will show three colors

bipyramidal: two pointed ends creating a double pyramid shape; sapphire most often in this shape

birefringence: double refraction; breaking up of light rays as they travel through the stone

blade: long and flat crystals, sometimes resembling a blade of grass; can grow quite long

botryoidal: interlocking, rounded masses that sometimes look like grapes or bubbles

bulk diffusion treatment:


chatoyancy: the cat’s eye-like phenomenon caused by light traveling down tiny fiber-like inclusions that act like a focal aperture, opening and closing when rotated under a direct light source; only one long ray is created

contact zone: the area where intruding magma or hot water contacts the pre-existing rock, where it melts and cools; where many gemstone crystals are formed

cryptocrystalline: miniscule crystals so small and compressed that you can't tell one from the other, the mass ending up having no resemblance to the the crystal forms from which it is made

crystal: a large formation built upon an orderly arrangement of atoms that, if reduced to its most basic size, will still maintain the same shape that we see in the larger version

crystal axes: the direction in which light travels through a stone; that gemstones have more than one axis providing a direction of light travel

crystal structure:

crystal systems: separated by their various shapes or type of

crystalline form:


cubic system: the most symmetrical of all crystal systems, with bold geometric shapes; isotropic (single refractive) · cube (6 sides) · octahedron (8 sides) · dodecahedron (12 sides) dichroic: anistropic stones that show two colors; type of pleocroism

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

diffusion treatment:

directional properties:

double refractive: meaning a light ray will split into two or more rays as it passes through a stone; anistropic

epigenetic inclusions: those that form after the big crystal has finished growing, usually due to a re-heating of the host crystal, when the big crystal gets so hot that part of it melts; new inclusions form inside the re-heated larger crystal after the big crystal has stopped growing

flint nodule: fossilized remains of an ancient sea sponge, whose body was almost pure silicon, that was covered up with sediment

fluorescence: property of certain gemstones where they are able to slow down ultrviolet rays enough to be visible to the human eye

hexagonal system: six equal, parallel sides longer than the ends; uniaxial.

inert: having no chance, movement, or reaction

igneous rock: rock formed where lava pushed its way to the surface; a source of gem bearing rock

imitations: materials used to mimic a gemstone without having the same composition as the stone it is imitating. ex: Synthetic sapphire used to imitate Alexandrite.



infrared light:

interference/strain colors: evidence of internal strain that appears as rainbow-like colors (primarily reds and oranges) under a polariscope

intergrowth: when three or more crystals of the same species grow together and interlock during the formation period

isomorphous replacement:

isomorphic transition:

isotropic: meaning a stone will show only one color; single refractive; stones of the cubic system are isotropic

labradorescence: the phenomenon displayed by labradorite (caused by lamelar fromation of lattice structure), which has an appearance of an predominantly bluish or greenish oil slick on water

limestone: a mostly calcium carbonate sedimentary rock

marble: a metamorphic rock created when a certain type of limestone is subjected to tremendous heat and pressure, usually due to magma intrusion

marine fossil: an ancient sea creature which has been preserved in sedimentary rock; flint is an example of a marine fossil

massive: no crystal structure

metamorphic rock: when host rock becomes so hot and so compressed that it melts and reforms as another mineral; upon cooling the original rock changes its form and becomes another type of rock

monoclinic system: crystals look uneven or asymmetrical from any angle, with even the somewhat triangular terminations lacking symmetry; biaxial.

opalescence: an opal’s ever-changing pattern of colors, caused by tiny sphere formations of silicon

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: the actual point at which a light ray splits in a double refractive stone and determines whether a stone is uniaxial or biaxial; appears as a cross-hair, bulls-eye, or double arrow figure; common instrument used to view the figures is the conoscope

optic sign:

orthorhombic system: four equal sides with a distinctive shape, looking like a box that has been crushed from the side; conical termination that looks like a cathedral dome; biaxial

pegmatite dike: a place where magma, or sometimes a hot water solution caused by the magma, has pushed into a pre-existing 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: those that existed before the larger crystal started growing. The big crystal grows and engulfs another in the process

railroad lines:


refractive index:

rhomboidal: lopsided, having unequal sides and no right angles

rock: a combination of several minerals

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)

spot reading method: finding the refractive index of a cabochon cut stone by finding where the light in the refracractometer appears as half light/half dark on the reading scale

strain/interference colors: evidence of internal strain that appears as rainbow-like colors (primarily reds and oranges) under a polariscope


syngenetic inclusions: those that grew at the same time but at a slower rate than the larger crystal in which they are found; the more slowly growing crystal becomes engulfed in the larger crystal and stops growing once inside the big crystal

synthetics: Man made gemstones having the same composition as the natural minerals. Lab grown.

table: the flat top of a faceted stone

tabular: grows as layers that are usually fairly easy to separate

tetragonal system: rectangular crystals with four equal sides sides and angles being longer than the ends are wide; uniaxial

trichroic: anistropic stones that may display three colors are; type of pleocroism

triclinic system: no equal sides or angles; generally form tabular crystals; biaxial

trigonal: 6 even sides longer than the ends, with some of the edges smoothed and poorly defined; distinctly flat surfaces on termination; uniaxial.

twinning: two of the same species grow together during the formation period; can interlock or grow from the edge outward

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; harmful to the eyes, protective eyewear a must

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