GIA GG key terms

The four factors used to describe and evaluate a polished diamond: color, clarity, cut, and carat weight. They form the basis of diamond grading and are used together to assess overall quality and value.

The unit of weight for diamonds and other gemstones. One carat equals 200 milligrams. Carat weight is one of the 4Cs and does not by itself indicate quality or value.

A unit used to express diamond weight below one carat. One point equals one-hundredth of a carat, so 100 points make one carat. A 0.25 ct diamond weighs 25 points.

A rating of how free a diamond is from internal inclusions and surface blemishes. The scale runs from Flawless to Included. Clarity grading uses 10x magnification under controlled lighting.

A internal characteristic of a diamond, such as a crystal, feather, or cloud. Inclusions are used to determine clarity grade and can also help identify a stone. They are inside the stone.

A surface irregularity on a polished diamond, such as a scratch, nick, or abrasion. Blemishes are external features and are considered in clarity grading along with internal inclusions.

A rating of a diamond's body color on a scale from D (colorless) to Z (light yellow or brown). Colorless stones are rarer and generally more valuable. Grading is done under controlled lighting against master stones.

A set of diamonds with known color grades used as comparison references when color grading a diamond. The stone being graded is compared to these stones under specific lighting and viewing conditions.

An assessment of how well a diamond's proportions, symmetry, and polish interact to return light. Cut grade affects brightness, fire, and scintillation. It is one of the 4Cs and applies to round brilliant diamonds.

The amount of light returned to the viewer's eye from a diamond. It is a key component of cut quality and depends on proportions and polish. Also called brilliance in some contexts.

The dispersion of white light into spectral colors as it reflects inside a diamond. Fire is seen as flashes of color and is influenced by cut. It is one of the three main aspects of cut quality.

The flashes of light and pattern of light and dark areas seen in a diamond when it or the viewer moves. Scintillation is a component of cut quality and is affected by facet arrangement and proportions.

A small magnifying lens used to examine gemstones. A 10x loupe is standard for diamond clarity grading. Proper use involves holding the stone close to the lens and using good lighting.

The large flat facet on the top of a polished diamond. Table size is expressed as a percentage of the diamond's average girdle diameter and affects light return and cut grade.

The narrow band around the widest part of a polished diamond, separating the crown from the pavilion. Girdle thickness is described in terms like thin, medium, or thick and can affect durability and cut grade.

The upper part of a polished diamond, above the girdle and including the table. Crown angle and height influence brightness and fire. It is one of the main parts of a faceted stone.

The lower part of a polished diamond, below the girdle, ending at the culet. Pavilion depth and angle are critical for light return. A poorly proportioned pavilion can cause a window or fish-eye effect.

The small facet at the bottom of a polished diamond, where the pavilion facets meet. Culet size is described as none, very small, small, medium, etc. A large culet may be visible through the table.

A measure of how much a gemstone bends light entering it from air. It is a key optical property used in gem identification. Higher refractive index generally means more brilliance.

An instrument that measures a gemstone's refractive index. It is used in gem identification to narrow down possibilities. Contact liquid and a polished facet are required for an accurate reading.

The difference between the maximum and minimum refractive indices of a doubly refractive gemstone. It is a measurable property used in identification. Amorphous or singly refractive stones have no birefringence.

An instrument with two polarizing filters used to detect double refraction and strain in gemstones. It helps separate singly refractive from doubly refractive stones. It is a standard tool in gem identification.

The ratio of a gemstone's weight to the weight of an equal volume of water. It is a useful identifying property. Specific gravity can be estimated with heavy liquids or measured with a balance.

Dense solutions used to estimate a gemstone's specific gravity by seeing whether it sinks, floats, or remains suspended. They are used in gem identification. Safety precautions are required because some liquids are toxic.

An instrument that splits light into its spectrum so absorption lines can be observed. These patterns help identify certain gemstones, especially colored stones. It is used in advanced gem identification.

The pattern of dark lines or bands seen when light passing through a gemstone is dispersed. It results from selective absorption of wavelengths. The pattern can help identify the gem species or coloring agent.

The display of different colors or shades in different directions in a doubly refractive colored gemstone. It is observed with a dichroscope or polariscope. Pleochroism can help identify certain species.

A small instrument used to view two directions of light through a gemstone at once. It helps detect pleochroism and can assist in identifying colored stones. It is commonly used in gem identification.

The basic color of a gemstone, such as red, blue, or green. Hue is one of the three dimensions of color used in colored stone grading. It is described with a modifier when needed, such as slightly purplish red.

The lightness or darkness of a gemstone's color. Tone is one of the three dimensions of color in colored stone grading. It is described on a scale from very light to very dark.

The strength or intensity of a gemstone's color. Saturation is one of the three dimensions of color in colored stone grading. It ranges from brownish or grayish to vivid.

A pearl formed when a nucleus or piece of mantle tissue is surgically implanted into a mollusk, which then deposits nacre around it. Cultured pearls are farmed rather than natural. They are the most common type in jewelry.

The iridescent substance secreted by a mollusk that forms around a nucleus or irritant to create a pearl. Nacre thickness affects pearl durability and luster. It is composed mainly of calcium carbonate and conchiolin.

The quality of light reflected from a pearl's surface. Luster is a key value factor in pearl grading and ranges from dull to excellent. High luster gives pearls a bright, reflective appearance.

A pearl formed without human intervention when an irritant enters a mollusk and nacre is deposited around it. Natural pearls are rare and often more valuable than cultured pearls. They are distinguished from cultured pearls by X-ray examination.

A cultured pearl produced by the Akoya oyster, typically small and round with high luster. Akoya pearls are often white or cream and are popular in strands and stud earrings. They are one of the main cultured pearl types.

A cultured pearl grown in the Pinctada maxima oyster, usually large and often white, gold, or silver. South Sea pearls are known for their size and thick nacre. They are among the most valuable cultured pearls.

A cultured pearl produced by the black-lipped oyster, often with dark colors such as green, blue, or aubergine. Tahitian pearls are valued for their exotic colors and large size. They are not necessarily black.

The metalwork that holds a gemstone or pearl in a piece of jewelry. Common styles include prong, bezel, and channel settings. The setting affects the stone's appearance, security, and wearability.

A setting that uses metal claws to hold a gemstone in place. It exposes much of the stone to light, enhancing brilliance. Prong settings are common in engagement rings but require periodic checking for loose stones.

A setting in which a metal rim encircles the edge of a gemstone. It provides strong protection and is common for softer stones. Bezel settings can make a stone appear slightly larger.