The 4Cs of diamond quality

Cut: the most important of the 4Cs

Cut grade measures how well a diamond's facets interact with light, not its shape, but its proportions, symmetry, and polish. GIA grades round brilliant cut diamonds on a scale from Excellent to Poor. An Excellent cut grade means the diamond was fashioned to maximise brilliance, fire, and scintillation from its specific rough. Cut is considered the most important of the 4Cs because even a flawless, perfectly colourless diamond looks dull if it is poorly cut (GIA; AGS).

The proportions that determine cut quality for round brilliants: table percentage (the table facet as a percentage of average girdle diameter, ideal 54-58%), depth percentage (total depth as a percentage of average girdle diameter, ideal 59-62.5%), crown angle, pavilion angle, and girdle thickness. GIA's cut grading algorithm weighs these proportions against measured light performance data to produce the cut grade.

Colour: D to Z

GIA grades diamond colour on a scale from D (completely colourless) to Z (light yellow or brown). The scale starts at D because earlier systems used A, B, C and other designations inconsistently; GIA started fresh at D in the 1950s to establish a universally understood standard. D-F are colourless; G-J are near-colourless (the most commercially significant range, where the colour is barely detectable by a non-gemologist); K-M are faint yellow; N-Z are increasing yellow or brown (GIA Gem Reference Guide, 2006).

Colour appearance depends on the setting: yellow gold makes near-colourless diamonds appear whiter by contrast; white gold or platinum shows colour more honestly. Most engagement ring buyers target the G-H range, near-colourless appearance at significantly lower prices than the D-F range.

Clarity: FL to I3

Clarity grades assess the size, nature, number, and position of internal characteristics (inclusions) and surface blemishes. GIA's clarity scale: Flawless (FL), Internally Flawless (IF), Very Very Slightly Included (VVS1, VVS2), Very Slightly Included (VS1, VS2), Slightly Included (SI1, SI2), and Included (I1, I2, I3). All graded at 10x magnification under defined conditions. Most engagement ring buyers target VS2-SI1, where inclusions are either invisible to the naked eye (VS2, SI1) or visible only with effort (SI1 in some cases). Eye-clean SI1 is the most popular clarity tier for value-conscious buyers.

Carat weight

One metric carat equals 0.2 grams. Carat weight affects price non-linearly: a 2-carat diamond is worth more than twice a 1-carat diamond of equivalent cut, colour, and clarity because large diamonds are rarer per carat than small diamonds. Popular "magic sizes" (0.50, 0.75, 1.00, 1.50, 2.00 ct) command slight premiums over diamonds just below those thresholds, a 0.98ct diamond is typically significantly cheaper than a 1.00ct of identical cut, colour, and clarity with no visible size difference.