Diamond Technical Terms with Definitions & Sources
Shopping for a diamond means navigating a world of technical terminology, from the 4 C's to cut grades, from pavilion depth to fancy shapes. Understanding these terms is essential to making an informed decision and getting the diamond that's right for you.
This glossary provides clear definitions for key diamond terms, each with references to authoritative sources like GIA (Gemological Institute of America) and IGI (International Gemological Institute). Whether you're encountering these terms for the first time or need a quick reference while comparing diamonds, this resource is designed to give you confidence and clarity.
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Term |
Definition |
Source & Link |
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4 C's |
The four characteristics used to evaluate diamond quality: Carat, Colour, Clarity, and Cut. This grading system was established to create standardized diamond evaluation. |
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Carat (ct) |
The unit of weight measurement for diamonds and gemstones. One carat equals 0.2 grams or 200 milligrams. |
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Carob Seeds |
Seeds from the carob tree (Ceratonia siliqua) that ancient gem traders used as counterweights for measuring gemstones, believing they had uniform weight. The word "carat" derives from this practice. |
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Colour Grade (Diamond) |
A measure of how colourless a diamond is, graded on a scale from D (colourless) to Z (light yellow or brown). The evaluation is conducted under controlled lighting conditions comparing the diamond to master stones. |
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D Colour |
The highest colour grade for diamonds, indicating a completely colourless stone with no detectable colour when viewed face-down through the pavilion. |
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E Colour |
The second-highest colour grade for diamonds, indicating a colourless stone with minute traces of colour that are difficult to detect even by trained graders. |
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F Colour |
Colour grade indicating a colourless diamond with slight colour detectable only by expert gemologists under controlled conditions. |
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G-H Colour |
Colour grades in the "near colourless" range where slight colour may be noticeable when compared to higher colour grades but appears colourless when viewed alone. |
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Fancy Colour Diamond |
Diamonds that exhibit intense, vivid colour outside the normal D-Z range (such as blue, pink, yellow, green). These are graded on a different scale based on colour intensity. |
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Clarity |
The evaluation of internal and external imperfections (inclusions and blemishes) in a diamond, graded under 10x magnification. |
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Inclusions |
Internal characteristics or imperfections within a diamond, including crystals, feathers, clouds, and other naturally occurring features formed during crystallization. |
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Blemishes |
External imperfections on the surface of a diamond, such as scratches, nicks, or polish marks. |
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FL (Flawless) |
Highest clarity grade indicating no inclusions or blemishes visible under 10x magnification by a skilled grader. |
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IF (Internally Flawless) |
Clarity grade indicating no inclusions visible under 10x magnification, with only insignificant surface blemishes. |
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VVS1, VVS2 (Very Very Slightly Included) |
Clarity grades indicating minute inclusions that are extremely difficult for a skilled grader to see under 10x magnification. VVS1 inclusions are slightly more difficult to see than VVS2. |
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VS1, VS2 (Very Slightly Included) |
Clarity grades indicating minor inclusions that are difficult to see under 10x magnification. VS1 inclusions are slightly more difficult to see than VS2. |
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SI1, SI2 (Slightly Included) |
Clarity grades indicating noticeable inclusions under 10x magnification that may be visible to the naked eye, particularly in SI2 grades. |
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I1, I2, I3 (Included) |
Clarity grades indicating obvious inclusions visible under 10x magnification and to the naked eye, which may affect the diamond's transparency, brilliance, and durability. |
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Eye-Clean |
Industry term describing a diamond whose inclusions are not visible to the unaided eye when viewed from a normal viewing distance (approximately 6-12 inches). |
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10x Magnification |
The standardized magnification level (10 times actual size) used by gemologists to grade diamond clarity using a jeweler's loupe. |
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Loupe |
A small magnification device used by jewelers and gemologists, typically providing 10x magnification for diamond examination and grading. |
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Cut |
How well a diamond's facets interact with light, determined by the proportions, symmetry, and polish of the diamond. Cut is the only C determined by human craftsmanship rather than nature. |
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Cut Grade |
GIA's assessment of a diamond's light performance and visual appearance, graded as Excellent, Very Good, Good, Fair, or Poor. Note: GIA only provides cut grades for round brilliant diamonds. |
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Excellent Cut |
Highest cut grade indicating superior light performance with exceptional brilliance, fire, and scintillation. |
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Very Good Cut |
Second-highest cut grade indicating very good light performance with slightly less optimal proportions than Excellent. |
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Light Performance |
How effectively a diamond interacts with light, encompassing brightness, fire, and scintillation. Well-cut diamonds return maximum light to the viewer's eye. |
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Light Return |
The amount of light that enters a diamond and returns to the observer's eye, determining the diamond's brightness and brilliance. |
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Crown |
The top portion of a diamond above the girdle, consisting of the table, star facets, bezel facets, and upper girdle facets. |
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Pavilion |
The bottom portion of a diamond below the girdle, designed to reflect light back through the crown to create brilliance. |
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Girdle |
The widest perimeter of a diamond, forming the separation plane between the crown and pavilion. |
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Table |
The largest facet on the top (crown) of a diamond, typically octagonal in round brilliants. |
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Facet |
A flat, polished surface on a diamond. The number, arrangement, and precision of facets determine how light interacts with the diamond. |
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Proportions |
The relationship between the diamond's dimensions and angles, including depth percentage, table percentage, crown angle, and pavilion angle. Proper proportions are critical for optimal light return. |
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Depth Percentage |
The height of a diamond (from table to culet) divided by its average diameter, expressed as a percentage. Affects light performance and how large the diamond appears. |
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Table Percentage |
The width of the table facet expressed as a percentage of the diamond's average diameter. Affects the balance between brilliance and fire. |
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Symmetry |
The exactness of shape and placement of facets in a diamond. Poor symmetry can negatively impact light performance. |
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Polish |
The quality and smoothness of a diamond's surface finish. Poor polish can diminish brilliance and light return. |
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Round Brilliant |
The most popular diamond shape with 58 facets (including the culet), mathematically optimized for maximum light return. The only shape for which GIA provides a cut grade. |
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Fancy Shape |
Any diamond shape other than round brilliant, including oval, princess, emerald, cushion, radiant, pear, marquise, and heart. |
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Oval Cut |
An elongated brilliant-cut diamond with an elliptical outline, typically containing 56-58 facets. Ovals have more surface area than rounds of the same carat weight. |
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Emerald Cut |
A rectangular step-cut diamond with cut corners, featuring parallel facets that create a "hall of mirrors" effect. Requires high clarity due to its transparent faceting. |
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Cushion Cut |
A square or rectangular brilliant-cut diamond with rounded corners and larger facets, resembling a pillow. Available in various faceting patterns. Also available as an elongated cushion. |
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Radiant Cut |
A rectangular or square brilliant-cut diamond with trimmed corners, combining the step-cut appearance of emerald cuts with the brilliance of round diamonds. |
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Step Cut |
A cutting style featuring rows of facets that are parallel to the girdle, arranged like staircase steps. Used in emerald and Asscher cuts. |
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Brilliant Cut |
A cutting style designed to maximize light return through triangular and kite-shaped facets that radiate from the center. Used in rounds, ovals, and many other shapes. |
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Bowtie Effect |
A dark bow-tie shaped shadow that appears across the center of some elongated fancy shapes (ovals, pears, marquises) due to light leakage. Well-cut stones minimize this effect. |
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Face-Up Appearance |
How a diamond looks when viewed from directly above (the normal viewing position when worn), as opposed to viewing from the side or pavilion. |
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Surface Area |
The visible area of the diamond's crown when viewed face-up. Shapes like ovals and marquises have more surface area than rounds of equal carat weight. |
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Length-to-Width Ratio |
The proportion of a fancy shape diamond's length divided by its width, affecting the overall shape and appearance. Each shape has preferred ratio ranges. |
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GIA (Gemological Institute of America) |
The world's foremost authority in gemology, established in 1931. GIA created the 4 Cs grading system and provides diamond grading reports. |
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IGI (International Gemological Institute) |
An independent gemological laboratory founded in 1975 that provides diamond grading reports, particularly common for lab-grown diamonds. |
About IGI |
| Lab-Grown Diamond |
A diamond created in a controlled laboratory environment using technological processes (CVD or HPHT) that replicate natural diamond |
Lab-Grown Diamonds |
