To the untrained eye, the differences between two diamonds might go unnoticed — but when the prices of those same gemstones vary by hundreds or even thousands of dollars, an explanation is called for. Today diamonds are assessed by the strictest of standards, developed over years by professionals within the industry. Each diamond is graded individually and valued according to the rarity of its characteristics. The main factors have become known in the trade as the 4 Cs: Colour, Clarity, Cut and Carat Weight.
Colour: the D-to-Z ladder
Diamonds are most often thought of as colourless, but they most commonly occur slightly yellow, brown or grey. Colour grading in the normal range measures how close to colourless a stone appears — differences so subtle they go unnoticed without practice and training. The most recognised scale, devised by the Gemological Institute of America, runs from D (devoid of colour, truly colourless) through E and F (extremely/very slight tint), G–J (slight tints), K–M (noticeable), down to N–Z (obvious tints). Generally the more colourless the diamond, the more valuable.
Diamonds occur in every colour. When a stone has enough colour intensity to be attractive — or a distinct tint other than yellow, brown or grey — it leaves the normal range and is classified as fancy-coloured. The rarest, red, blue and pink, can command the highest prices of all; a 5.03-carat pink at USD 5.3–6.6 million and an 8.01-carat blue at USD 11.5–14 million are the seminar's reference cases. When buying a coloured diamond, ask for a certificate of authenticity from a well-known gemmological laboratory stating the colour is natural: many beautiful colours today are treated, and treated stones should not command natural-colour prices.
Colour grading itself demands controlled conditions: specific lighting, a proper viewing background, comparison stones, viewing direction — and a grader in the right condition. The professional cautions read like a checklist of human factors: colour blindness, rested eyes, caffeine and other substances, and morning grading only.
Clarity: the fingerprint
Clarity assesses the internal characteristics (inclusions) and external characteristics (blemishes) a diamond possesses. It is rare for a diamond to be completely free of them — that would be Flawless — and those with the least are the most highly prized. Grading is done with 10x magnification on the GIA-style scale: Flawless, Internally Flawless, then VVS1–2 (very, very slight), VS1–2 (very slight), SI1–2 (slightly included), and I1–3 (included), with industrial-quality 'bort' below the gem scale. Each diamond is like a fingerprint: characteristics differ in type, size, shape, quantity, position and relief, so no two diamonds are ever the same — a fact the trade has turned into many an important sale.
Carat weight: from carob seeds to points
One of the oldest units of weight was a seed from the carob plant — small, reasonably uniform, and readily available in the gem-trading areas. Standardisation came by 1930 with the metric system: one metric carat = 0.200 gram (5 carats = 1 gram), the standard unit for diamonds and most gemstones. Below a carat, weight is divided into points: 1 carat = 100 points, so 0.50 ct = 50 points, and precision is recorded to the hundredth (0.67 cts). Because large sizes are rare, per-carat price does not rise proportionally: a 2-carat diamond costs substantially more than twice a 1-carat — until sizes above ~20 carats, where stones become collectors' and museum pieces rather than jewellery.
Cut: style and make
Cut covers two areas: the cutting style and outline shape, and the finish — the dimensions and proportions of the finished stone. The most popular style is the brilliant cut, whose name refers to the faceting style, not the shape: triangular and kite-shaped facets radiating from the centre, giving us the oval brilliant, marquise brilliant, pear brilliant and more. Less common are the step cut (step-like facets around the stone) and the mixed cut, which combines both on one gem.
Put together, the 4 Cs are not a scorecard but a rarity ladder — each factor measuring how far a stone sits from the common case, and the price following the rarity.
Grounded in the JDMIS Gemmology (GT104 Value Factors in Diamonds) seminar materials. Learn diamond grading hands-on in Gem Identification Methods and Gemmology: Secrets of the Gem and Jewellery Trade. Related reading: Natural vs Synthetic Diamonds and How to Read a Gem Certificate.