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Glossary

A

Average girdle diameter

The result achieved by adding the smallest and largest girdle measurements of a round brilliant and dividing by two.

B

Baguette

A small, four-sided step cut that's rectangular, square or tapered

Blemish

Clarity characteristic that’s confined to the surface of a polished gemstone.

Branded cut

Cutting style that’s developed, named, and promoted by a specific manufacturer.

Brightness

The effect of all the diamond’s internal and external reflections of white light.

Brilliant Cut

Cutting style with triangular or kite-shaped facets that radiate from the center toward the girdle.

C

Carat

The international unit of measurement for gem weight. One carat equals 1/5 of a gram (0.200 g).

Clarity characteristics

The collective term for inclusions and blemishes.

Clarity Plot/Plot

A color-keyed diagram of a gemstone’s significant clarity characteristics.

Colored diamonds

Yellow, brown, and gray diamonds with more color than the Z masterstone, or that exhibit any other color face-up.

Craftsmanship

The care that goes into the fashioning of a polished diamond, as confirmed by its finish.

Crown

Top part of the faceted gem above the girdle

Crown angle

The angle formed by the bezel facets and the girdle plane.

Culet

Small facet at the bottom of a finished gem

D

Depth

The distance from the table to the culet of a polished gem.

Depth of color

The combination of tone and saturation that determines how noticeable a color is.

Dispersion

The separation of white light into spectral colors.

Durability

A gemstone's ability to withstand wear, heat and chemicals.

E


F

Facet

A Flat, polished surface on a finished gem

Feather

A collective term for diamond cleavages and fractures.

Feature

A characteristic or part of a piece of jewelry.

Finish

The quality of the polish and precision of the cut of a fashioned gemstone.

Fire

The flashes of color your see in a polished diamond.

Fluorescence

Emission of visible light by a material when it’s exposed to ultraviolet radiation.

Four Cs

Four value factors (clarity, color, cut and carat weight) that describe the quality of a finished diamond.

Fracture filling

Treatment that involves injecting a molten glass substance into a diamond’s surface-reaching feathers or laser drill-holes.

G

Girdle

The narrow section of a finished gem that forms the boundary between the crown and the pavilion and functions as the gem’s setting edge.

Girdle outline

Face-up shape of a polished gem.

H

Hardness

How well a gem stone resists scratches and abrasion.

Hue

Your first impression of a color; the basic color of an object.

I

Inclusion

Clarity characteristic totally enclosed in a polished gemstone or extending into it from the surface.

Irradiation

A treatment that changes the color of a gem by exposing it to radioactive materials.

J


K

Karat

A unit of measure for the fineness of gold, abbreviated “K” or “Kt.”

L

Laser drilling

Treatment that involves using a concentrated beam of laser light to reach a diamond’s dark inclusions and to disguise or eliminate them.

Length-to-width ratio

Comparison of the length to the width of a fancy-shaped diamond.

Light carat

A trade term for a diamond that weighs between 0.96 ct. and 0.99 ct.

Light half

A trade term for a diamond that weighs between 0.45 ct. and 0.49 ct.

Loupe

A small, portable magnifying lens used for examining gemstones.

Loupe clean

Term that describes a diamond that might have surface blemishes, but shows no inclusions under 10X magnification.

M

Make

The qualities of a faceted diamond’s proportions and finish.

Masterstones

A set of color comparison diamonds that defines GIA’s diamond color grades in the normal (D-to-Z) range.

Melee

Very small faceted diamonds.

Metric carat

The international unit of measurement for gem weight. One carat equals one-fifth of a gram (0.200 g).

Mixed cut

Cut that combines brilliant-cut and step-cut styles.

N

Near-colorless

A general term for diamonds in the G-to-J color range.

Normal color range

Range of diamond colors from colorless to light yellow, brown, and gray.

O


P

Pavilion

Lower part of a faceted gem below the girdle.

Pavilion angle

The angle formed by the pavilion main facets and the girdle plane.

Pavilion depth percentage

The distance from the bottom of the girdle plane to the culet, expressed as a percentage of the average girdle diameter.

Per-carat price

The price of a gem divided by its weight.

Piqué

A general term for included stones.

Point

One hundredth of a carat (0.01 ct.).

Polish

The overall condition of the facet surfaces of a finished diamond.

Proportions

The angles and relative measurements of a polished gem, and the relationships between them.

Q


R

Refraction

The change in speed and possible change in direction of light as it travels from one transparent material to another.

Refractive index (RI)

A measure of the change in the speed and angle of light as it passes from one material to another.

S

Saturation

A color’s strength or intensity, ranging from a dull hue to a pure, vivid hue.

Scintillation

The flashes of light and the contrasting dark areas you see when the diamond, the light, or the observer moves.

Shoulder

One of the two sides adjacent to the rounded end of a pear or oval shape.

Simulant

A natural or manmade material that resembles a gem (also called imitation).

Single cut

A round stone with 17 or 18 facets.

Stability

How well a diamond resists chemicals and temperature changes.

Standard round brilliant

A round brilliant-cut stone with 57 or 58 facets. Often called a full cut.

Steam cleaner

A machine that cleans jewelry with highpressure steam.

Step cut

A cutting style that features long, narrow facets in rows parallel to the girdle on both the crown and pavilion.

Symmetry

The exactness of a finished gem’s shape and the placement of its facets.

Synthetic gem

A manmade gem with essentially the same chemical composition, crystal structure, and properties as a natural gem.

T

Table percentage

Table size expressed as a percentage of a round brilliant’s average girdle diameter.

Tone

A color’s degree of darkness or lightness.

Total depth percentage

Table-to-culet depth, expressed as a percentage of average girdle diameter

Total gem weight

The combined weight of all the stones in a piece of jewelry that contains a variety of gems.

Total weight

The combined weight of all the diamonds in a piece of jewelry that only contains diamonds

Toughness

How well a gemstone resists breaking and chipping.

 

U

Ultrasonic cleaner

A machine that cleans jewelry with highfrequency sound waves in a liquid solution.

Ultraviolet

Light wavelength that’s invisible to the human eye.

V


W

Wing

One of the two sides near the point of a marquise, pear, or heart shape.

X


Y


Z



Source: GIA Text book - Diamond Essentials