Glossary
A
Anneal
A heating enhancement process used with color gemstones.
B
Bar channel setting
Individual metal bars separate the gemstones. The metal is molded around a gem to lock it in place.
Bezel setting
A metal rim gently secures a gemstone in place by the girdle. The bezel setting guards the gem from damage, yet does not block light from creating Brilliance.
Blemish
A flaw (blister or spot) on the surface of a gem.
Brilliance
White light reflected up through the surface of a diamond. Brilliance is maximized by cutting a diamond to the correct proportions.
Brilliant cut
Any cut with 58 facets. A Brilliant cut can have several shapes: round, oval, radiant (basically square), as well as pear-shaped or heart-shaped.Birthstone
Birthstones have their roots in ancient astrology, and there have been many
birthstone lists used over the years.
C
Cabochon
A gemstone shape resembling a dome. Onyx is commonly cut in a cabochon shape, as are star sapphires.
Carat
A unit of weight for a diamond, equivalent to 200 milligrams, or 0.2 gram.
Center stone
The central, dominant stone in a piece of jewelry set with multiple stones. In a ring set with one stone, the center stone is also called the solitaire.
Chemistry
It is the chemistry of a gem material which is ultimately responsible for all of its physical properties. Chemistry determines color which in turn determines value. Heat treatment is a chemical modification of a gem material to improve it's color and value.
Channel setting
Ridges in the metal create a channel that holds gemstones in place.
Clarity
The clarity scale is used to describe how few inclusions a gemstone has. For diamonds, the clarity scale ranges from flawless (FL), meaning a gemstone has no internal or external flaws, to severely included (I3), meaning there are many flaws clearly visible to the unaided eye.
Cloud
A cluster of small inclusions inside a gemstone.
Color
The color scale is used to describe the color tones of a gemstone. For diamonds, the color scale ranges from D, meaning completely colorless, to Z, which indicates a distinct yellow cast. As the scale moves from D to Z, it indicates increasing levels of yellow and /pown tone. For color gemstones, the color is measured by tone and hue. Color is the most important criteria in determining the value a colored gemstone. More important than clarity and cut, even subtle differences in hue can correspond with marked variations in valuations
Comfort fit
The rounded finish on the inside of a ring's band. This design does not pinch the skin of the finger as much as other ring bands and provides comfort for constant wear.
Corundum
A hard mineral that exists in the form of colored crystals, such as rubies and sapphires.
Crown
The edges of a gemstone above the girdle and surrounding the table. Colored light escapes through the crown in the form of fire.
Crystal System (Group)
Crystals are divided in to 7 systems, each corresponding to a specific set of angular, geometrical, and symmetrical specifications. Every gem group can be classified according to its crystallization. The study of crystallization is invaluable in the determination and identification of rough gem materials.
Culet
The facet at the tip of the pavilion of a gemstone. The culets of older, European-cut diamonds can be seen through the table of the diamond and appear as round, glassy disks. The preferred culet is visible only as a tiny focal point where the facets of the pavilion converge.
Cut
Refers to the geometric proportions of a gemstone. The cut of a gemstone is one of the most important factors in determining how much sparkle a gemstone produces.
Cut, ideal
A diamond cut of perfect proportions, having depth percentages and table percentages that maximize fire and Brilliance.
Cut, very good
A grade of diamond cut that fits very strict requirements for depth percentage and table percentage. These specific proportions maximize fire and Brilliance in a diamond.
Cut, good
The cut grade assigned to diamonds with acceptable proportions. These diamonds generally have very good Brilliance and fire and they are also are beautiful in jewelry.
Cut, fair
The cut used to maximize the weight of a diamond, generally at the expense of fire and Brilliance. Diamonds with this cut are less expensive than diamonds having good and very good cuts, and they lack the sparkle people expect from a diamond.
Cut, poor
This cut makes diamonds appear lifeless. These diamonds are not recommended for fine jewelry.
Cut , european
The style of diamond cutting popular from approximately 1890 to the 1930s. Unlike the old mine cut preceding it, the European cut has a round girdle (perimeter) made possible by the introduction of the power /puting machine (/puting is the term for shaping the girdle of a diamond, the first step in the cutting process). The European cut can be distinguished by the size of the table (the top, flat facet) in relation to the diameter of the stone. In a European cut, the table is smaller in relation to the diameter of the stone. Also, the culet (the bottom facet, is often large, often appearing to create a hole at the bottom of the diamond, when viewed from the top, since the large culet lets light escape instead of reflecting back to the viewer.
D
Depth
The height of a gemstone measured from the culet to the table.
Diameter
The width of the diamond, as measured across the widest part of the girdle.
Depth percentage
The height of a gemstone, measured from the culet to the table, divided by the width of the gemstone. The depth percentage is critical to creating Brilliance and fire in a diamond, and a gemstone with a depth percentage too low or too high will lack sparkle.
Doublet
A form of gemstone trickery that was devised to allow inexpensive materials to imitate the more valuable gemstones before modern synthetics were available. A doublet can take several forms but always involves a fake gemstone produced by gluing together two different materials to form an illusion. A very common one in Victorian times was the garnet and glass doublet. This involved a red garnet top, glued to a colored glass bottom. The refractive properties of a faceted stone are such that the red of the garnet only shows at odd angles, or if the stone is immersed in a special liquid with a high refractive index. Thus, for example, a green glass bottom with a garnet top will give the appearance of a fine emerald because the top is a natural gemstone with cut facets, and a few natural imperfections, and the bottom is /pight green which reflects throughout the stone. The effect is hard to appreciate unless you've seen one.
Density
Water has a density of 1gram/cc. Density is the weight of a material as compared to an equal volume of water.
E
Enhancements
Treatments, such as heating and oiling, that are an accepted part of the process of gemstone cutting and polishing. The AGTA only approves of enhancements that are permanent, do not counterfeit the beauty of the gem, and only tap the natural potential beauty of the gem.
Eye-clean
A gemstone with no inclusions visible to the unaided eye.
F
Facet
The flat polished surfaces on a gemstone. A round, Brilliant-cut diamond has 58 facets.
Fire
The colored light reflected from within a diamond through the crown. Fire is maximized by cutting a diamond to the correct proportions.
Fluorescence
A glow, usually of a bluish color, that emanates from some diamonds when exposed to ultraviolet light. Strong fluorescence is not desirable, but faint fluorescence does not affect the appearance of a diamond.
Foilback
A method of coating the back of a stone with silver, gold, or colored foil. This enhances the Brilliancy of the stone, by reflecting back as much light as possible. It is commonly seen in costume jewelry. A foilbacked rhinestone whose foil has been damaged (often from water creeping in) does not sparkle anymore and is said to be a "dead" stone, lowering the value of the piece. Before, modern, highly reflective cuts were developed, even diamonds were foilbacked.
G
GIA
The Gemological Institute of America, or GIA, is recognized as a top authority on grading gemstones.
Girdle
The narrow band around the widest part of a diamond. The setting in a piece of jewelry usually grasps the diamond around the girdle.
Grade
Gemstones are frequently sent to an independent laboratory such as the GIA, AGS, NGL, or AGTA to be graded against a master set of gemstones, according to an industry-wide common set of grading guidelines. These grades are compiled to form a complete appraisal which offers an assessment the worth of a gemstone.
H
Head
The part of the setting that holds the center stone or solitaire in place.
Hardness
The part of the setting that holds the center stone or solitaire in place. Moh's scale of hardness is useful for comparing the relative hardness of different gem materials. The values are as follows:
- 10 – Diamond (hardest)
- 9 – Corundum
- 8 – Topaz
- 7 – Quartz
- 6 – Feldspar
- 5 – Apatite
- 4 – Fluorite
- 3 – Calcite
- 2 – Gypsum
- 1 – Talc (softest)
Head shape
The shape of the head, which is determined by the shape of the gemstone the head holds. For example, the head that holds an ideal-cut diamond is round, where as a head that holds a princess-cut diamond is square.
Head-size range
The range of carat weights of gemstones that can be mounted into a particular head.
Hue
A term used to describe the attributes of a gemstone's color.
I
Inclusion
The trace minerals, fractures, and other imperfections that contribute to the unique fingerprint of a gemstone. All but the rarest gemstones contain inclusions, which are created during the gemstones' formation in the earth.
M
Millimeter
The unit of measure used to determine a gem's diameter, equal to about 0.04 inch.Mohs hardness scale
A scale developed by Friedrich Mohs to determine the relative hardness of minerals and other objects. The scale assigns the numbers 1 to 10 to specific minerals, softest to hardest respectively, using a scratch test.
Marquise
A stone cut in a boat shape, pointed at both ends, with rounded sides. Note that the correct pronunciation is "Mar-KEYS", not "Mar-KEY" which is commonly heard.
Mine Cut
A style of diamond cutting popular before 1890 or so, it features a cushion shaped outline, rather than the round outline of the modern cut and old European cuts, and has a different facet arrangement.
O
Opaque
Description of a gemstone that is so thick with inclusions or flaws that light is blocked from passing through the gemstone.
P
Pear-shaped
Description of a shape of gems and pearl rounded on one end and tapered on the other.
Point
A weight measure equal to 0.01 carat. (A 0.50-carat diamond is said to weigh 50 points.)
Polish
Polish describes how smoothly the facets of a diamond have been polished, whether any marks are visible from the polishing wheel, and how defined the edges of each facet are. Since these characteristics are rarely visible to the unaided eye, for an excellent value, look for a polish grade of very good or good, though the higher grades of ideal and excellent are also recommended. Avoid diamonds with polish grades of fair or poor, as the quality of their polish may affect the Brilliance of the diamond.
Paste
A term for imitation gemstones. Fine jewellery was often imitated in finely made copies to protect the wearer from theft, and these were referred to as "paste".
Pavé
(pah-VAY) a gem setting technique in which the stones are set low and very closely spaced, so that the surface appears to be paved with gemstones. Most commonly seen with diamonds, but may be used with any stone.
Pleochroism
(from Greek pleion, "more," and chros, "colour"), in optics, the selective absorption in crystals of light vi/pating in different planes. Pleochroism is the general term for both dichroism, which is found in uniaxial crystals (crystals with a single optic axis), and trichroism, found in biaxial crystals (two optic axes). It can be observed only in coloured, doubly refracting crystals.
R
Refractive Index
The ratio of the velocity of light in air to its velocity in the new substance is known as the refractive index of that substance. Almost every gem material has it's own unique specific refractive index (R.I.). Determination of the RI is probably the most conclusive measurement available to gemologists
S
Saturation
This term refers to the color purity, or the degree to which the gem color is free from /pown or gray hues. The most desirable gemstones, which show little gray or /pown, are often described as having vivid or strong color saturation.
Shape
The geometrical form a gemstone takes on after it has been cut and polished. The most common shapes of gemstones are round, oval, radiant, princess, square, as well as pear-shaped or heart-shaped.
Side stone
A gemstone set alongside, or as part of a group of gemstones encircling a center stone.
Sparkle
The combination of fire and Brilliance; the amount of light reflected from a diamond.
Symmetry
Symmetry refers to the alignment of the diamond facets - whether they are the same size on each side, and whether or not the facets on the pavilion are directly below the facets on the crown.
T
Table
The largest facet of a gemstone, located on the top of the gemstone, through which refracted light escapes as Brilliance.
Table percentage
The width of the table divided by the diameter of the gemstone. The table percentage is critical to creating sparkle in a gemstone; a gemstone with a table percentage too low or too high will lack sparkle.
Tone
A classification scale, ranging from light to dark, which indicates color intensity.
U
Uniformity
The grading system used to denote how well pearls in a piece of jewelry match one another. Uniformity can be excellent, good, or fair.
