A

ABRASION – Small scratches on the surface of a diamond or colored gemstone. In a diamond, these are usually white in color and caused by contact with other diamonds. See also Plotting Diagram. In a colored gemstone, they are caused by contact with various other materials.

ABSORPTION SPECTRA – A reflection of the electromagnetic radiation absorbed by a colored diamond or colored gemstone over a visible range of wavelengths (approximately 400-800 nanometers). They are recorded on a graph that plots the transmittance against wavelengths. And they can help identify the species and/or variety of stone, as well as the color origin of colored diamonds.

ASSEMBLED STONE – A stone consisting of a combination of components held together by bonding material. These components can include natural or synthetic colored gemstones (of similar or mixed variety), glass, and/or other manufactured products.

ASTERISM – An optical phenomenon in which star-like rays are visible in a colored gemstone. This is caused by the reflection of light from minute, oriented, and aligned needle-like inclusions.

 

B

BAGUETTE – A rectangular style of step cut used for diamonds and colored gemstones.

BI-COLORATION – A phenomenon in which two colors are visible in a colored gemstone.

BLEACHING – A topical treatment, typically using acids or hydrogen peroxide, that lightens and/or evens out hue. Bleaching is used on select colored gemstones, including jadeite and coral, and pearls.

BLEMISH – A surface feature on the exterior of a diamond or colored gemstone, such as an abrasion, natural, nick, or scratch. A blemish can affect the finish of a stone. For diamonds, see also Plotting Diagram.

BRILLIANT CUT – The most common style of diamond cutting, also used for some colored gemstones, traditionally consisting of a combination of triangular, octagonal, and kite-shaped facets. See also Cut (Shape and Style).

BRUISE – A minute imperfection that breaks the surface of a diamond or colored gemstone. For diamonds, see also Plotting Diagram.

BRUTED GIRDLE – On a diamond or colored gemstone, a girdle that is cut, but unpolished.

 

C

CABOCHON – A polished cut colored gemstone of domed form (having a convex surface). See also Cut (Shape and Style).

CARAT WEIGHT – A unit of metric measurement used for diamonds and colored gemstones. One carat (ct.) equals 100 points, 200 milligrams, or 1/5 of a gram. The chart below illustrates carat weights for diamonds.

CAVITY – An isolated opening that breaks the surface of a diamond or colored gemstone. For diamonds, see also Plotting Diagram.

CHATOYANCY – An optical phenomenon, also known as the Cat’s Eye effect, in which a narrow band or line resembling a cat’s eye is visible in a cabochon-cut colored gemstone. This is caused by the reflection of light from minute (typically parallel) inclusions.

CHIP – A shallow, jagged surface break on a diamond or colored gemstone. For diamonds, see also Plotting Diagram.

CLOUD – In a diamond or colored gemstone, a cloud-like, semi-transparent area created by minute pinpoint inclusions. On EGL USA reports, it is plotted for diamonds (see also Plotting Diagram) and noted for colored gemstones.

COLOR CHANGE (CHANGE OF COLOR) – In some diamonds, known as chameleons, a phenomenon in which their color changes when exposed to certain intensities of heat or when kept in darkness for an extended period of time. In some colored gemstones, a phenomenon in which their hue appears to vary when exposed to different types of light.

COLORED GEMSTONE – A gemstone distinguished by its hue or by a combination of hue, tone, and saturation. Typically, colored gemstones exhibit single colors. More unique phenomena — such as bi-coloration, color-change, play of color, asterism, or chatoyancy — can create distinctive stones.

COMPOSITE STONE – A stone created from a minimal amount of natural or synthetic colored gemstones mixed with other manufactured products. The resulting stone, typically, has the appearance of a colored gemstone, with very little actual colored gemstone content.

COUNTRY OF ORIGIN – The presumed geographical source of a colored gemstone, discoverable only for particular stones with unique identifying characteristics.

CROWN – The part of any faceted diamond or colored gemstone above the girdle. See also Proportions.

CRYSTAL – For a diamond or colored gemstone, a visible crystalline structure of variable transparency on the interior of the stone. This can occur naturally or as a result of a treatment, and typically serves as an identifying characteristic. On EGL USA reports, it is plotted for diamonds (see also Plotting Diagram) and noted as an identifying characteristic for colored gemstones.

CRYSTAL WITH HALO – On a colored gemstone, a cloudy, irregular feature with a glowing border, created when a crystal bursts during heat treatment. A crystal with halo can be an identifying characteristic noted on an EGL USA report.

CULET – The small facet polished across what would otherwise be the sharp point or tip of the pavilion of a faceted diamond or colored gemstone. This is described on a range from none (a sharp point) to extremely large (an extended, flatter surface). For diamonds, see also Proportions.

CUSHION CUT – A diamond or colored gemstone shape in which the outline is square or rectangular, with rounded corners and/or bowed sides. See also Cut (Shape and Style).

CUT (SHAPE AND STYLE) – A description of the silhouette or form created by the contours and facets of a diamond or colored gemstone. Shapes vary from round to fancy cuts, such as cushion, emerald, heart, marquise, oval, pear, princess, and triangle. And style includes variations of brilliant, step, mixed, and, for colored gemstones, cabochon cuts. Beautiful stones can be found in virtually any shape or style.

Sample Shapes

   

CUSHION

EMERALD

HEART

MARQUISE

OVAL

PEAR

PRINCESS

ROUND

TRIANGLE

Sample Styles

   

 

Top (Crown) View      Side (Pavilion) View

BRILLIANT

STEP

MIXED

CABOCHON

 

D

DIFFUSION – On a colored gemstone, the introduction of chemicals combined with high temperatures to create color enhancing or asterism effects. Diffusion can impact color throughout a gem or merely on its surface. Like other surface treatments, the latter tends to be less permanent. Diffusion is used on stones such as feldspars, rubies, and sapphires.

DISPERSION – The separation of white light into its component spectral colors. See also Fire.

DURABILITY – A combination of hardness, toughness, and stability that describes the ability of a specific diamond or colored gemstone to resist wear.

DYEING – On a colored gemstone, the application of coloring agents to alter, intensify, or improve consistency of hue. In pearls, the use of chemicals or dyes to change the nacre’s color or overtone. The results of this process can range from distinct color transformations to subtle overtone shifts. EGL USA reports clearly identify dyed colored gemstones and pearls.

 

E

EMERALD CUT – A diamond or colored gemstone shape in which the outline is rectangular with cut corners, and the facets are rectangular and trapezoidal. See also Cut (Shape and Style).

EXTRA FACET – An additional small, flat surface on a diamond or colored gemstone. For diamonds, see also Plotting Diagram.

 

F

FACET – One of the small, flat surfaces that is polished on a diamond or colored gemstone.

FACETED GIRDLE – On a diamond or colored gemstone, a girdle that is cut and polished for maximum surface reflection, with many flat planes around the circumference of the stone.

FANCY CUT – A diamond or colored gemstone shape other than round. See also Cut (Shape and Style).

FEATHER – A fracture in a diamond or colored gemstone that typically breaks the surface. For diamonds, see also Plotting Diagram.

FEATHER FILLING – Also known as fracture filling, a treatment that introduces a glass-like material in the natural feathers or fractures of a diamond or colored gemstone.

For diamonds, this process enhances the appearance of clarity. While the treatment is stable for normal wear, it is not considered permanent. In most cases, should damage to the treatment occur, the diamond can be repaired and its enhancement restored. EGL USA clearly identifies such stones with laser inscriptions, along with special Enhanced Diamond Reports. For diamonds, see also Plotting Diagram.

For colored gemstones — such as alexandrites, emeralds, rubies, and tourmalines — feather filling can also enhance the appearance of clarity. In addition, it can improve durability.

FINGERPRINT – An identifying characteristic in a diamond or colored gemstone, similar in appearance to a feather.

FIRE – Flashes of rainbow or spectral colors seen in diamonds or colored gemstones as a result of dispersion.

FLUORESCENCE – The capacity of diamonds, colored gemstones, and pearls to emit visible light when exposed to higher energy wavelengths. In diamonds and colored gemstones, this occurs when their atoms react to long- and short-wave ultraviolet rays. In pearls, this occurs when some of their elements react to x-rays. Fluorescence is measured for identification purposes and described on a scale from inert (none) to very strong. Its presence can also help to confirm a pearl’s cultured origin.

FRACTURE FILLING – See Feather Filling.

FULL CUT – A description of a brilliant cut, round diamond or colored gemstone with 57-58 facets.

 

G

GEMSTONE – A mineral, rock, organic, or inorganic material that is, typically, cut and polished for use in jewelry. There are dozens of types of gemstones — including diamonds, colored gemstones, and pearls — each with a unique set of physical and optical properties.

GIRDLE – The narrow band around the perimeter of a polished or faceted diamond or colored gemstone. A girdle separates the crown from the pavilion facets and will vary in thickness depending on a stone’s individual style and shape. Thickness is measured on a scale from extremely thin to extremely thick, with mid-range thickness being preferred. A girdle can be further described as bruted, faceted, polished, or a combination of these. See also Proportions.

GRAINING – Lines or patterns visible at the juncture between two crystals, formed either on the surface or within a diamond or colored gemstone. On EGL USA reports, it is plotted for diamonds (see also Plotting Diagram) and noted for colored gemstones.

 

H

HARDNESS – The ability of a diamond or colored gemstone to resist scratching. A diamond is the hardest specimen in nature. See also Mohs Scale.

HEART CUT – A diamond or colored gemstone shape in which the outline is like its name: heart-shaped. See also Cut (Shape and Style).

HEATING – On a colored gemstone, the application of controlled high temperatures to alter or improve its color and/or internal structure. Heat enhances many types of gemstones. And it achieves this by essentially continuing the process started by nature. For example, heat can turn a naturally brown tanzanite into a striking blue/violet, or transform a cloudy ruby to a distinct red. Pearls can also be treated with high temperatures to modify color. However, heat is often used with other treatments, such as bleaching, to maximize effects.

HUE – The primary impression of color such as red, green, or blue.

 

I

IDENTIFYING CHARACTERISTICS – Physical aspects of a diamond or colored gemstone that help to confirm its singularity or categorization. These can range from inclusions (fingerprints, needles, etc.) to modifiers caused by treatments (crystals with halos, reduced silks, etc.). In addition, identifying characteristics can refer to instrument-based measurements such as refractive index, x-ray fluorescence, infrared spectra, Raman spectra, or specific gravity.

IMPREGNATION – The application of a colorless agent — such as wax, oil, resin, or other polymer material — into a porous colored gemstone. Impregnation is used to improve the durability and appearance of stones such as jades, opals, and turquoises.

INCLUSION – An internal feature on a diamond or a colored gemstone, such as a cavity, crystal, feather, internal graining, pinpoint, etc. On EGL USA reports, it is plotted for diamonds (see also Plotting Diagram) and, when serving as an identifying characteristic, noted for colored gemstones.

INFRARED LIGHT – Part of the electromagnetic spectrum that is not visible to the human eye, between approximately 800 nanometers (longer than red in the visible spectrum) to 1 millimeter (bordering microwave).

INFRARED SPECTRA – A representation of the vibrational modes of a diamond’s or colored gemstone’s molecules over a range of wavelengths outside of the visible spectrum. They are recorded on a graph that plots intensity against wavelengths. And they can help identify the species and/or variety of stone.

INTERNAL GRAINING – See Graining.

IRRADIATION – The use of radiation to alter the appearance of a diamond, colored gemstone, or pearl. On a diamond or colored gemstone, radiation such as neutron or electron bombardment can change the internal structure and, therefore, the stone’s color and perceived clarity. This process is often followed by heating, which can further refine the results. Irradiation can create vibrant pink, yellow, green, and green/blue hues in diamonds, as well as beautiful blue topazes and red tourmalines. On certain pearls, such as freshwater or Akoya pearls, gamma ray treatments can generate darker gray, blue, or black colors.

 

L

LEAKAGE – A phenomenon that occurs in diamonds and colored gemstones when light entering a stone fails to reflect back through its crown, dispersing instead through the pavilion.

 

M

MARQUISE CUT – A diamond or colored gemstone shape in which the girdle outline is elliptical, with pointed ends. See also Cut (Shape and Style).

MEASUREMENTS – For round diamonds and colored gemstones, an indication of maximum-minimum diameter x depth, in millimeters. Fancy shapes are indicated by length x width x depth. For round pearls, an indication of diameter, in millimeters. Other pearls are measured by length x width x depth. Measurements of pearl strands or jewelry are described as a range, average, or graduation from maximum to minimum.

MELEE – A term used primarily to describe small faceted diamonds or colored gemstones of approximately .12 carat or less.

MIXED CUT – A style of diamond or colored gemstone cutting that features aspects of both brilliant and step cutting. See also Cut (Shape and Style).

MOHS SCALE – A loose scale of hardness, used for field collecting, which allows for identification of specimens. Devised by Friedrich Mohs, a German mineralogist, in the 19th century. The comparative scale of hardness is as follows:

1)    Talc
2)    Gypsum
3)    Calcite
4)    Fluorite
5)    Apatite
6)    Feldspar
7)    Quartz
8)    Topaz and Beryl
9)    Corundum
10)  Diamond

MOUNTING – The portion of a piece of jewelry in which a gemstone or other object is set.

 

N

NEEDLE – A thin inclusion of variable length, in a diamond or colored gemstone. On EGL USA reports, it is plotted for diamonds (see also Plotting Diagram) and noted as an identifying characteristic for colored gemstones.

 

O

OPAQUE – A description of a characteristic of a diamond or colored gemstone that is neither transparent nor translucent, so it does not transmit light.

OVAL CUT – A diamond or colored gemstone shape in which the girdle outline is elliptical or oval. See also Cut (Shape and Style).

 

P

PAVILION – The portion of a faceted diamond or colored gemstone that lies below the girdle. See also Proportions.

PEAR CUT – A diamond or colored gemstone shape in which the girdle outline is teardrop- or pear-shaped. See also Cut (Shape and Style).

PHOSPHORESCENCE – The capacity of a diamond or colored gemstone to emit visible light for a variable period of time after exposure to long- and short-wave ultraviolet rays.

PINPOINT – Inside a diamond or colored gemstone, a minute, circular inclusion. On EGL USA reports, it is plotted for diamonds (see also Plotting Diagram) and noted for gemstones.

PLAY OF COLOR – In a colored gemstone, a phenomenon in which its color appears to flash or change as the stone is tilted. This is caused by diffraction and can be seen most easily in opals.

POINT – A measurement in the weight of a diamond or colored gemstone equal to 1/100 of a carat. Thus, 0.50 carats is equal to 50 points.

POLISH LINES – Small parallel lines left on the surface of a diamond’s or colored gemstone’s facets by a rotating polishing/grinding wheel. For diamonds, see also Plotting Diagram.

POLISHED GIRDLE – On a diamond or colored gemstone, a girdle that has been cut and polished to yield a uniform, highly reflective surface. See also Girdle.

PRINCESS CUT – A diamond or colored gemstone shape in which the outline is either square or rectangular, with pointed corners. See also Cut (Shape and Style).

PROPORTIONS – On a diamond or colored gemstone, the dimensions and facet angles, and the relationship between them. On EGL USA diamond reports, proportions are noted or diagrammed. See also Measurements.

Diameter
Table

Crown Angle
Pavilion Angle

Crown

Girdle

Depth

Pavilion

Culet

 

R

RAMAN SPECTRA – A representation of the vibrational modes of a diamond’s, colored gemstone’s, or pearl’s molecules as they react to a monochromatic (laser) light source. For diamonds, the Raman spectra measure photo luminescence to detect treatments to the stone. For all stones, Raman spectra are recorded on a graph that plots intensity versus frequency. And they can help identify the species and/or variety of stone, as well as any applied treatments.

REDUCED SILK – A structurally changed needle, caused by heat treatment in a colored gemstone. On an EGL USA report, this is noted as an identifying characteristic.

REFLECTION – The return of light that strikes the surface of a diamond or colored gemstone. This effect can also occur when light strikes specific inclusions within a stone.

REFRACTION – The change in direction of a ray of light as it enters a diamond or colored gemstone.

REFRACTIVE INDEX – The degree to which visible light bends as it passes through a diamond, colored gemstone, or pearl. Each type of gemstone exhibits a unique refractive index (RI) or RI range — a result of its distinct chemical composition and physical crystallization. As such, RI is a strong identifying characteristic.

ROUGH – Any uncut or unpolished diamond or colored gemstone.

ROUND CUT – A diamond or colored gemstone shape in which the outline is circular. See also Cut (Shape and Style).

 

S

SATURATION – An attribute of color that denotes its strength.

SCRATCH – On the surface of a diamond or colored gemstone, a white, narrow, shallow marked caused by abrasion. For diamonds, see also Plotting Diagram.

SHAPE OF COLORED GEMSTONES – See Cut (Shape and Style).

SILK – A naturally occurring, short needle in a colored gemstone. On an EGL USA report, this is noted as an identifying characteristic.

SINGLE CUT – Traditionally, a round cut for a diamond or colored gemstone with 17 facets: nine crown facets (including the table) and eight pavilion facets.

SOLITAIRE – A piece of jewelry containing a single diamond or colored gemstone.

SPECIES – A group of gemstones that share similar chemical composition and physical structure. Examples of species include beryl and corundum. See also Variety.

SPECIFIC GRAVITY – The ratio of the density of a solid or liquid to the density of an equal volume of water (or for a gas, to an equal volume of hydrogen).

STABILITY – Ability of a diamond or colored gemstone to maintain its integrity under normal conditions.

STEP CUT – A style of diamond- or colored gemstone-cutting that creates straight facets that run parallel to the girdle and decrease in size as they move further from it (resembling steps), as in an emerald cut. See also Cut (Shape and Style).

SURFACE COATING – On a diamond, colored gemstone, or pearl, the application of a thin artificial layer on the stone’s exterior to impact appearance. Coatings are, typically, not considered permanent; their effects may diminish over time.

On a diamond, chemical coatings can disguise less desirable interior hues. On a colored gemstone, coatings can improve color and overall appearance. And on a pearl, coatings can create the illusion of a smoother surface and/or enhanced luster. On an EGL USA report for any stone type, coatings are clearly noted. In addition, EGL USA does not provide color or clarity grades for coated diamonds.

SURFACE GRAINING – See Graining.

SUSTAINABLE ORIGIN – Sustainable origin refers to the environmentally-friendly source of a diamond or colored gemstone and, if applicable, its related jewelry material. This can include heirloom and reclaimed items, as well as newly mined items that are conflict-free and ethically-sourced. EGL USA’s Envira Diamond Reports™ and Envira Colored Gemstone Reports™ can verify sustainable origin.

SYNTHETIC COLORED GEMSTONE – A man-made stone created through various processes. Synthetic colored gemstones have essentially the same chemical, physical, and optical properties as natural colored gemstones.

 

T

TABLE – The horizontal, top flat facet on the crown of a faceted diamond or colored gemstone. See also Proportions.

TONE – An attribute of color that denotes its lightness or darkness.

TOUGHNESS – The ability of a mineral or gemstone to resist breakage. Jadeite is the toughest gemstone due to an interlocking molecular structure that helps to minimize its weak points.

TRANSPARENCY – The amount of light transmitted through a diamond or colored gemstone. It is influenced by the texture of the material itself and the presence of inclusions. Transparency is rated on a scale of transparent (typically preferred), semi-transparent, translucent, semi-translucent, and opaque.

TREATMENT – The application of processes or agents to enhance the perceived color, clarity, phenomena, or durability of a diamond, colored gemstone, or pearl. Treatments are routinely applied to stones, with measurable results. Treatments are disclosed on EGL USA reports.

For diamonds, see also Annealing, Feather Filling, High Pressure and High Temperature (HPHT), Irradiation, Internal Laser Drilling, Laser Drilling, and Surface Coating.

For colored gemstones, see also Bleaching, Diffusion, Dyeing, Feather Filling, Heating, Impregnation, Irradiation, and Surface Coating.

For pearls, see also Bleaching, Dyeing, Heating, Irradiation, and Surface Coating.

TRIANGLE CUT – A three-sided diamond or colored gemstone shape in which the outline is like its name: triangular. See also Cut (Shape and Style).

 

U

ULTRAVIOLET LIGHT – Light that is invisible to the naked eye because it consists of wavelengths shorter than those of visible light.

 

V

VARIETY – A subcategory of species, used to further define stones based on common color or phenomena. The beryl species, for example, includes emerald and aquamarine varieties. And corundum includes ruby and sapphire varieties. See also Species.

 

W

WISP – A curvy, hair-like feature on a diamond or colored gemstone that can appear as an isolated element or in a cloud-like form.

 

X

X-RAY FLUORESCENCE – Also known as XRF, the capacity of a diamond, colored gemstone, or pearl to emit energy via secondary x-rays when exposed to a high-energy incident (primary) x-ray. The intensity of the emitted x-rays indicates the stone’s chemical elements for identification purposes.

Note: All photos and diagrams above are intended as general guides.
Renderings may vary with computer settings.