Abrasion: Tiny nicks along facet junctions, producing white fuzzy lines instead of sharp crisp facet edges.
Bezel: A facet on the Crown, or upper part of the Diamond above the Girdle.
BezelFacets: The eight large, four-sided facets on the crown of a round, brilliant-cut gem, the upper points of which join the table and the lower points, the girdle. Some diamond cutters further distinguish four of these as “quoin” or “top-corner” facets.
Brilliance: The brightness that seems to come from the very heart of a diamond. It is the effect that makes diamonds unique among all other gemstones. While other gemstones also display brilliance, none have the power to equal the extent of diamond's light-reflecting power. Brilliance is created primarily when light enters through the table, reaches the pavilion facets, and is then reflected back out through the table, where the light is most visible to your eye.
Brilliant Cut: One of three styles of faceting arrangements. In this type of arrangement, all facets appear to radiate out from the center of the diamond toward its outer edges. It is called a brilliant cut because it is designed to maximize brilliance. Round diamonds, ovals, radiant, princesses, hearts, marquises, and pears all fall within this category of cut.
Carat Weight: The metric carat, which equals 0.200 gram, is the standard unit of weight for diamonds and most other gems. If other factors are equal, the more a stone weighs, the more valuable it will be.
Cavity: An inclusion consisting of a large or deep opening in the stone.
Clarity: A stone's relative position on a flawless to imperfect scale. Clarity characteristics are classified as inclusions (internal) or blemishes (external). The size, number, position, nature, and color or relief of characteristics determines the clarity grade. Very few diamonds are flawless, that is, show no inclusions or blemishes when examined by a skilled grader under 10X magnification. If other factors are equal, flawless stones are most valuable.
Cloud: A group of tiny white inclusions which result in a milky or cloudy appearance.
Colour: Grading color in the normal range involves deciding how closely a stone's body color approaches colorlessness. Most diamonds have at least a trace of yellow or brown body color. With the exception of some natural fancy color, such as blue, pink, purple, or red, the colourless grade is the most valuable.
Crown: The upper part of the diamond above the girdle. Crown consists of a large flat area on top called a table, and several facets below it.
Culet: The smallest facet at the bottom of the diamond.
Crown angle: The angle at which a diamond's bezel facets (or, on emerald cuts, the row of concentric facets) intersect the girdle plane. This gentle slope of the facets that surround the table is what helps to create the dispersion, or fire, in a diamond. White light entering at the different angles is broken up into its spectral hues, creating a beautiful play of color inside the diamond. The crown angle also helps to enhance the brilliance of a diamond.
Culet: A tiny flat facet that diamond cutters sometimes add at the bottom of a diamond's pavilion. Its purpose is to protect the tip of the pavilion from being chipped or damaged. Once a diamond is set in jewelry, though, the setting itself generally provides the pavilion with sufficient protection from impact or wear. Large or extremely large culets were common in diamonds cut in the early part of this century, such as the Old European or Old Mine Cut. However, such large culets are rarely seen today. Most modern shapes have either no culet at all, or a small or very small culet.
Cut: The proportions and finish of a polished diamond (also called make). Cut can also mean shape, as in emerald cut or marquise cut. Proportions are the size and angle relationships between the facets and different parts of the stone. Finish includes polish and details of facet shape and placement. Cut affects both the weight yield from rough and the optical efficiency of the polished stone; the more successful the cutter is in balancing these considerations, the more valuable the stone will be.
Depth: The height of a diamond from the culet to the table. The depth is measured in millimeters.
Depth Percentage: Depth percentage, which expresses how deep the diamond is in comparison to how wide it is. This depth percentage of a diamond is important to its brilliance and value. depth percentage that might be excessive for one diamond cut might be necessary for another type of cut. For example, a 75% or 78% depth in a princess cut diamond would be typical and quite attractive. However, a depth of even 65% would be unnecessary and even detrimental to a round diamond's beauty.
Diamond Cutting: The method by which a rough diamond that has been mined from the earth is shaped into a finished, faceted stone. As a first step, cleaving or sawing is often used to separate the rough into smaller, more workable pieces that will each eventually become an individual polished gem. Next, bruiting grinds away the edges, providing the outline shape (for example, heart, oval or round) for the gem. Faceting is done in two steps: during blocking, the table, culet, bezel and pavilion main facets are cut; afterward, the star, upper girdle and lower girdle facets are added. Once the fully faceted diamond has been inspected and improved, it is boiled in hydrochloric and sulfuric acids to remove dust and oil. The diamond is then considered a finished, polished gem.
Diamond Gauge: An instrument that is used to measure a diamond's length, width and depth in milli-meters.
Dispersion: Arranged around the table facet on the crown are several smaller facets (bezel and star facets) angled downward at varying degrees. These facets, and the angles at which they are cut, have been skilfully designed to break up white light as it hits the surface, separating it into its component spectral colours (for example, red, blue and green). This effect, which appears as a play of small flashes of color across the surface of the diamond as it is tilted, is what we refer to as the diamond's dispersion (also called "fire"). This play of color should not be confused with a diamond's natural body color (normally white, though sometimes yellow, brown, pink or blue in the case of fancy color diamonds) which is uniform throughout the entire diamond and is constant, regardless of whether it is being tilted or not.
Durability: The durability of a gem depends both on its hardness and “toughness.” It may be quite tough but easily scratched, or it may be exceedingly hard but lack toughness because of easy cleavage. Diamond is highest on the scale of hardness and, despite it rather easily developed octahedral cleavage, it is among the toughest of gemstones.
Facet: The smooth, flat faces on the surface of a diamond. They allow light to both enter a diamond and reflect off its surface at different angles, creating the wonderful play of color and light for which diamonds are famous. The table below shows all the facets on a round brilliant cut diamond. A round brilliant has 58 facets (or 57 if there is no culet).
Faceted Girdle: Sometimes cutters polish the girdle into 32 facets.
Fancy Cut: Any style of diamond cutting other than the round brilliant or single cut. Fancy cuts include the marquise, emerald cut, heart shape, pear shape, keystone, half moon, kite, triangle, and many others. Also called the “fancy-shaped” diamond or “modern cut.”
Feather: When the plane of cleavage or fracture in a diamond is viewed at right angle to it, the appearance is often reminiscent of a feather. Thus, cleavage and fractures are often called “feathers.”
Fire: Flashes of different spectrum colours seen in diamonds and other gemstones as the result of dispersion.
Fisheye: A diamond whose pavilion is exceedingly shallow, producing a glassy appearance and a noticeable dearth of brilliancy.
Flawless: The recommended term for a diamond without external or internal flaws or blemishes of any description when viewed by a trained eye under efficient illumination and under a corrected magnifier of not less than ten power; binocular magnification under dark-field illumination is preferred. The American Gem Society advocates the use of the term “flawless” by its members, while at the same time denying them the use of the term perfect. The Federal Trade Commission permits the use of the term “flawless,” but only if a stone conforms to its definition of the word perfect, without reference to make or color.
Fluorescence: The property of changing the wavelength of radiation to one in the visible range; for example, the visible wavelengths emitted by a material when excited by invisible radiation (such as X-rays, ultraviolet rays or cathode rays), as well as by certain visible wavelengths. It is exhibited by ruby, kunzite, yellow-green synthetic spinel, some diamonds and opals, and many other substances.
Fracture: The breaking or chipping of a stone along a direction other than a cleavage plane.
Girdle: The outer edge, or periphery, of a fashioned stone; the portion that is usually grasped by the setting or mounting; the dividing line between the crown and pavilion.
Girdle Facets: The 32 triangular facets that adjoin the girdle of a round brilliant-cut stone, 16 above and 16 below. Also called upper- and lower-girdle facets, upper- and lower-break facets, top- and bottom-half facets, skew facets or cross facets. Facets are sometimes placed directly on the girdle, in which case the stone is usually said to have a “faceted girdle,” to have a polished girdle or to be “girdle faceted.”
Girdle Reflection: When a diamond has a pavilion that is too shallow or flat, the girdle is seen reflected in the table.
Girdle Thickness: The width of the outer edge, or periphery, of a fashioned diamond or other gemstone. In a rounded style of cutting, such as the round brilliant or pear shape, the girdle edges, when viewed parallel to the girdle plane, consist of undulating lines caused by the intersection of the flat facets with the curved girdle. In such stones, the girdle thickness is measured across the midpoints of opposing upper- and lower-girdle facets.
Inclusion: A general term used to refer to any external blemish or internal inclusion or flaw on or in a fashioned diamond; e.g., a feather, carbon spot, knot, fissure, scratch, natural, etc. The term “flaw” and “imperfection” are usually used interchangeably.
Length-to-width ratio: A comparison of how much longer a diamond is than it is wide. It is used to analyze the outline of fancy shapes only; it is never applied to round diamonds. There's really no such thing as an 'ideal' ratio; it's simply a matter of personal aesthetic preferences. For example, while many people are told that a 2 to 1 ratio is best for a marquise, most people actually tend to prefer a ratio of around 1.80 to 1 when they actually look at marquises. And though the standard accepted range for the length-to- width ratio of a marquise generally falls between 1.70 to 1 and 2.05 to 1, there are customers who insist on having 'fatter' marquises of about 1.60 to 1 and other customers who want longer, thinner marquises of 2.25 to 1.
Symmetry: Refers to variations in a diamond's symmetry. The small variations can include misalignment of facets or facets that fail to point correctly to the girdle (this misalignment is completely undetectable to the naked eye). Symmetry is regarded as an indicator of the quality of as diamond's cut; it is graded as Ideal, Excellent, Very Good, Good, Fair or Poor.
Point: 100th of a carat.
Polish Lines: Tiny parallel lines left by polishing. Fine parallel ridges confined to a single facet, caused by crystal structure irregularities, or tiny parallel polished grooves produced by irregularities in the scarf surface.
Proportions: A term that meant originally the distribution of the mass of a fashioned diamond above and below the girdle. Use by diamond men has broadened its meaning to include the major factors that determine cutting quality; i.e., total depth as a percentage of the girdle diameter, table diameter, girdle thickness, facet angles, symmetry, and even details of finish.
Refraction: The bending of light rays. The deflection from a straight path suffered by a ray of light as it passes obliquely from a medium of one optical density to a medium of a different optical density, as from air into water or from air into a gemstone. The degree of bending is related to the change in velocity of light and the angel at which the light impinges.
Rough Girdle: A grainy or pitted girdle surface, often with nicks.
Scintillation: The display of reflections from the polished facets of a gemstone seen by the observer when either the illuminant, the gemstone or the observer is in motion—a flashing or twinkling of light from the facets.
Twinning Lines: Visible line on or with in a fashioned diamond, caused by twinning in the crystal. Since the orientation on one side of a twin plane differs from that on the other, the best polishing direction for one is a poorer one for the other; as a result, a line remains at the surface. Also called knot lines.