Diamond Glossary

This diamond glossary defines the key terms used when buying a diamond — from the 4Cs (cut, colour, clarity and carat) to certificate labs, fluorescence and the parts of a cut diamond. Each definition is written in plain English so you can shop with confidence.

4Cs

The four factors used to grade and price every diamond: Cut, Colour, Clarity and Carat weight. Cut has the biggest impact on sparkle, while carat weight has the biggest impact on price.

Carat

The unit of weight for a diamond, where 1 carat equals 0.2 grams. Carat measures weight, not size — a well-cut 1-carat round diamond is about 6.5mm in diameter.

Cut

How well a diamond is proportioned and faceted to reflect light. Cut is the most important of the 4Cs because it controls a diamond's brilliance, fire and sparkle. GIA grades round diamond cut from Excellent to Poor.

Colour

The absence of colour in a diamond, graded by GIA from D (completely colourless) to Z (light yellow). G-H grades look colourless once set and offer excellent value.

Clarity

A measure of the inclusions and blemishes in a diamond, graded by GIA on an 11-point scale from FL (Flawless) to I3 (Included), assessed under 10x magnification.

GIA

The Gemological Institute of America, the most trusted independent diamond grading laboratory and the body that created the 4Cs grading system. A GIA report is the global benchmark for diamond quality.

IGI

The International Gemological Institute, a widely used grading laboratory, especially for lab-grown diamonds. IGI reports certify a diamond's 4Cs and characteristics.

Grading Certificate

An independent laboratory report (also called a diamond certificate) that documents a diamond's 4Cs, measurements, proportions and any treatments. Always buy a certified diamond and verify the report number with the issuing lab.

Fluorescence

The soft glow some diamonds emit under UV light, graded from None to Very Strong. In most diamonds it has no visible effect, and faint blue fluorescence can make near-colourless stones look whiter.

Inclusion

A natural internal characteristic of a diamond, such as a crystal, feather or cloud, formed as the diamond grew. The number, size and position of inclusions determine the clarity grade.

Blemish

A surface characteristic of a diamond, such as a scratch, nick or extra facet. Blemishes affect the clarity grade but only the diamond's surface, not its interior.

Eye-clean

A diamond with no inclusions or blemishes visible to the naked eye at a normal viewing distance of about 20-30cm. Many VS and SI1 diamonds are eye-clean and offer the best value.

Brilliance

The white light reflected back from a diamond. Strong brilliance makes a diamond look bright and lively, and is driven mainly by cut quality.

Fire

The flashes of rainbow colour a diamond disperses when light passes through it. Fire is most striking in well-cut diamonds.

Scintillation

The sparkle and play of light and dark areas seen when a diamond, the light or the viewer moves. Good scintillation depends on cut and symmetry.

Hearts and Arrows

A pattern of eight symmetrical hearts and arrows seen in a round diamond with near-perfect cut and symmetry. It is a hallmark of super-ideal optical precision, though not an official GIA grade.

Round Brilliant

The most popular diamond shape, with 57 or 58 facets engineered for maximum brilliance and fire. It accounts for the majority of diamonds sold.

Fancy Shape

Any diamond shape other than round, such as oval, cushion, emerald, pear, princess, radiant, marquise, asscher or heart. Fancy shapes can appear larger per carat than round diamonds.

Lab-grown Diamond

A real diamond grown in a laboratory (by CVD or HPHT) with the same physical and chemical properties as a mined diamond. At the same 4Cs, lab-grown diamonds typically cost 80-90% less than natural diamonds.

CVD Diamond (Chemical Vapour Deposition)

A lab-grown method: carbon atoms from a hydrocarbon gas such as methane are deposited layer by layer onto a diamond seed in a vacuum chamber at around 800°C using microwave energy. Equipment costs more than HPHT but clarity is easier to control, and CVD is the current market majority. Under high magnification it can show faint striated growth lines.

HPHT Diamond (High Pressure High Temperature)

A lab-grown method: a carbon source is held at roughly 1,500°C and over 5 GPa, recreating the conditions under which natural diamonds form. It typically costs 2–3x more than CVD, and some stones show a fluorescence reaction under specific UV light.

Polish

The smoothness of a diamond's surface after final finishing, graded by GIA from Excellent to Poor. High polish lets light pass cleanly for maximum brilliance.

Symmetry

How precisely a diamond's facets are aligned and shaped, graded by GIA from Excellent to Poor. Good symmetry reflects light evenly for maximum sparkle.

Table

The large, flat top facet of a diamond. The table size, expressed as a percentage of the diameter, is a key proportion — around 54-57% is ideal for a round brilliant.

Crown

The upper part of a diamond, between the table and the girdle. The crown angle helps create fire and brilliance.

Pavilion

The lower part of a diamond, below the girdle, that tapers towards the culet. The pavilion angle is critical for reflecting light back to the eye.

Girdle

The thin band around the widest part of a diamond, where the crown meets the pavilion. It is often where a GIA laser inscription is placed.

Culet

The tiny facet or point at the very bottom of a diamond. A None or Small culet is preferred for a round brilliant.