achroic Without color; a colorless gemstone.
achromatic As above
adularescence A phenomenal effect; e.g., the billowing “moonglow” effect in moonstone.
akoya pearl A pearl from the saltwater akoya-gai oyster (Pinctada martensii); “Japanese pearl.”
baroque Any pearl that is not symmetrical, round, or teardrop, oval, or button shaped.
bead nucleation The use of a shell bead, usually spherical, implanted in the oyster to stimulate the growth of a cultured pearl, and forming the center of the pearl.
bellied Describing a stone purposely cut with extra weight around the girdle, yielding a bulbous outline.
bicolor A stone with two distinct, separate hues, common in tourmaline.
bleed color The loss of saturation and tone when the viewing environment is shifted between natural and incandescent lighting.
body color The color of light transmitted through a gem, as distinguished from key color, the color of refracted light.
brilliance The total quantity of light refracted and reflected (from a gemstone) back to the eye of the viewer.
brilliant cut Usually refers to a full-cut brilliant of fifty-eight facets, with thirty-two facets and table above the girdle, twenty-four facets and a culet below; used almost universally in cutting larger round diamonds.
byewater Off-color; poor color and transparency; see also water.
“buying the cert” A purchase based not on an analysis of the beauty of the stone, but on the language of the grading report (certificate).
cabochon, en cabochon Gem with a rounded top, without facets. French, “little head.”
carat Unit of weight in gemstones, one-fifth of a metric gram
cat’s-eye Phenomenal effect in cabochon cut gemstones resembling the iris of a cat’s eye.
clarity One of the “four Cs” of quality grading, referring to the presence or absence of inclusions or flaws.
color See body color, key color.
crown The top half of a faceted gemstone; the portion above the girdle.
crystal One of the “four Cs” of gem connoisseurship, coined by author, referring to the transparency and diaphaneity of the gem. See also water and transparency.
culet The point at the very bottom of the pavilion of a gemstone.
cut The style in which a gem has been fashioned; e.g., emerald cut, brilliant cut. Also refers to a gem’s proportions; e.g., well cut.
daystone A term coined by the author to describe a gem species or variety or single stone that looks its best in natural daylight.
diaphaneity The property of being transparent or translucent.
dichroic Of two colors; the characteristic of a transparent substance to divide refracted white light into two distinct rays.
diffraction The modification of white light as it breaks up into the color spectrum.
dispersion The division of white light into its constituent components as in light through a prism; the rainbow effect.
dog A poor quality gemstone in a parcel.
drusy Tiny quartz crystals growing on the surface of a gemstone.
en cameo Cut in relief. Opposite of cutting intaglio.
enhancement Any process applied to a gemstone to improve its color or clarity; also heat enhancement, burning, cooking; see also treatment.
extinction The dark gray to black portion of a face-up gemstone that does not refract light; usually caused by off-axis refraction.
eye The finest gemstone in a parcel.
eye-clean, eye-flawless Describing a gem with no inclusions when viewed with the naked eye (assumes 20/20 vision).
eye-visible Inclusions visible to the eye without magnification.
face up The view of a gem from the top or crown.
fancy color In diamond, any color other than colorless viewed face up; in diamond any color is a fancy color.
fish-eye A dark gray to black (achroic) spot at the center of the gem caused by improper proportions from poor cutting; see also extinction
flawless Describing a gemstone with no visible inclusions under 10X magnification.
flour Tiny inclusions that cause a “sleepy” effect in the key color of a gemstone.
fluorescence A glow or color visible under ultraviolet light.
fortification The technical term for stripes of color in chalcedony.
“four Cs” The four factors used to analyze and discuss the beauty of a gemstone: color, clarity, cut, and crystal. Altered by author. Traditionally, color, cut, clarity and carat (weight)
gamut limit The point on the tonal (light to dark) scale at which a given hue produces the most vivid saturation.
girdle The outer edge of a faceted stone, the area of greatest diameter, usually the part where the prongs are placed when the gem is set.
“holding the carat” A cutting strategy designed to produce a finished gem above a certain whole carat weight.
hue The descriptive technical term for color; e.g., red, pinkish orange, and chartreuse are hues.
ideal cut A specific set of proportions discovered by Marcel Tolkowsky in 1919 thought to produce the greatest brilliance and maximum dispersion in a round brilliant cut diamond.
imperial Archaic term used to describe certain hues of topaz.
incandescent Light produced by a flame, candle, campfire, or light bulb.
inclusion Anything visible to the naked eye or under magnification in a gemstone, such as a foreign body or crack. A stone with an inclusion is described as included. See also eye clean, loupe clean
intaglio Designs cut in a gemstone that appear in relief when the stone is pressed into a soft substance like clay. Opposite of cutting en cameo.
iridescence The exhibition of prismatic (rainbow) colors on the surface of a gem. See also orient, overtone.
kelvin A unit used to measure light temperature.
key color The color of the light refracted out of a faceted gemstone; the color of the gem’s brilliance or sparkle.
loupe clean Describing a flawless gem; no visible inclusions under 10X magnification.
luster Reflections off the surface of a gem or pearl.
mask A modifying color, usually brown or gray, that diminishes the saturation (brightness or vividness) of a gem’s hue.
master stone Stone of a known color and quality used for comparing other gems of the same species or variety; also a diamond of known color grade used to determine the grade of other diamonds.
modifier A color that changes the appearance (hue) of another, e.g., a primary hue such as red can be modified by a secondary hue such as orange, yielding an orangy red hue. Saturation modifer, see mask.
Mohs scale A relative scale of gem hardness; talc is 1, diamond is 10.
mosaic The complex visual scene in the face-up gem. See also multicolor effect.
multicolor effect The display of divergent colors or tonal variations of the same color on a gem viewed in the face-up position.
nacre The mother-of-pearl secretions of the mollusk; pearl essence.
nailhead Dark center in a gem; see fish-eye.
native cut A poorly proportioned gemstone supposedly fashioned by primitive means; considered pejorative.
nightstone A term adapted by the author to describe a gem species or variety that looks its best in incandescent light, showing its most saturated hue. Traditionally, an opal that retains a strong play-of-color in low light environments.
nonchromatic Without color; a colorless stone, without chroma; see also achromatic, achroic.
off-color Insufficient color, or saturation, in a diamond to be considered good color. Used to describe colorless diamond with a strong tint of yellow.
opaque Impenetrable by light; neither transparent nor translucent. Opacity is the quality of being opaque.
orient The iridescent effect visible in finer quality pearls; also called overtone.
overcolor A stone with a hue that is overly dark in tone, usually above eighty-five percent.
overtone See orient.
padparadscha Color of the lotus; a light to medium-toned pinkish orange to orangy pink sapphire.
pair Two gems matching in color, cut, clarity, crystal, and diameter.
parcel Gems sold as a group.
pavilion The portion of a faceted stone beneath the girdle.
Peruzzi cut An early brilliant style cut, the first brilliant.
phenomenal stone A stone that exhibits a phenomenal effect, such as a star, cat’s-eye, or adularescence.
pick A selection from a parcel of gemstones.
pink To heat topaz at a low temperature to turn the hue to pink.
play of color The iridescent effect in opal.
precious An archaic term used to describe certain hues in topaz.
primary When referring to hue, the dominant hue in a gemstone; a pinkish red gem has a red primary hue.
reflection Light reflected from a surface.
refraction The bending or deflection of light passing from one transparent media to another; e.g., from air to water, from a gem into the air.
refractive index A measure of the angle of the deflection of light as it passes from one substance to another.
reinforcement Alternating bands of color in chalcedony (agate). Coined by author to describe a memory aid used to recall the color of a specific gem, i.e. using taste to reinforce sight.
rutile A crystal (inclusion) that forms in golden hairlike masses.
saturation The quantity of color in a gem which translates into the color’s vividness or dullness.
scintillation The breaking up of light into tiny constituents, a function of the facets of a gem.
seal stone A gem carved in intaglio producing a design in relief, when pressed into clay, usually the owner’s signature or emblem.
secondary When referring to hue, the second or modifying color; an orangy red gem has an orange secondary hue.
simpatico An affinity between a certain color pearl and the skin of the potential wearer.
skylight Light diffused around the body of the viewer with the back turned to the sun.
sleepy Having a fuzzy or misty quality in the brilliance of a gem, a lack ofcrispness. Defining quality in Kashmir sapphire
smoky Dirty or sooty looking. Also refers specifically to grayish fancy color diamonds.
sooty Blackish and fuzzy or smoky, refers to the crystal or transparency of the gem.
star A phenomenal effect in some gemstones, appearing as a six rayed figure.
suite Three or more matched gems.
super-d A misnomer used to describe an ultra-transparent diamond. A noted characteristic of Golconda diamonds.
texture In transparent gems, a description of color zoning visible face up. In pearls, a reference to any indentations or imperfections in the pearl’s surface.
tint A hint of color, not sufficiently saturated to be a hue.
tissue nucleation The use of mantle tissue from a donor mollusk to stimulate the growth of a pearl. A tissue-nucleated pearl is a non-nucleated cultured pearl.
tone The third constituent of color, defined as lightness to darkness.
translucent Allowing light to pass through, but preventing the clear viewing of images.
transmission luminescence Emission of light effect by a substance caused by the excitation of light rays.
transparent Allowing light to pass through so that objects may be clearly seen.
treatment, heat treatment The heating of a gemstone to improve its color or clarity.
trichroic The breaking up of light into three constituent rays, each containing a portion of the visible spectrum (rainbow).
water An archaic term that refers to the combination of color and transparency in gemstones; used hierarchically: first water (gem of the finest water), second water, third water, byewater.
window The center of a gem cut too shallow. Produces a read through effect that lacks brilliance; also called lens effect

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