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 bright 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.
Fresh-water pearl
A pearl produced by a mollusk that inhabits freshwater, usually these pearls are shaped like an uneven grain of rice. There is also a variety called Tennessee fresh water pearls that taper like a long tooth.
Fob
A short chain with a decorative seal or other device attached to the end. The fob and chain hung outside watch pocket, and could be used to pull the watch out of the pocket.
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.
French Jet
Black glass fashioned to imitate real jet. Glass is heavier than real jet, and can feel cold to the touch compared to real jet.
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