The Grader
The 4Cs in Detail
Colour, Clarity, Cut and Carat — and the atomic story beneath them, where nitrogen and boron decide what a diamond can be.
Polished diamonds are graded on the 4Cs — Colour, Clarity, Cut and Carat. Terminology and classification are set by three institutions, GIA, IDC and CIBJO, each with its own rules. This guide works in GIA and IDC terms. Before the four, though, comes the question they all rest on: what is the diamond actually made of?
Two atomic families
Ideally a diamond is pure carbon. In practice, other elements slip into the crystal lattice as it forms and change what the stone can be. Diamonds divide into two atomic types by whether they carry nitrogen — and those types decide colour, rarity and value.
| Type | Nitrogen | Atomic structure | Typical colours | Occurrence |
|---|---|---|---|---|
| Ia | With N | Clustered nitrogen atoms | Colourless, yellow | ~98% |
| Ib | With N | Scattered nitrogen atoms | Yellow, orange, brown | ~0.1% |
| IIa | Without N | Lattice distortion (insulator) | Colourless; fancy pink, brown | ~1.8% |
| IIb | Without N | Boron atoms (semi-conductive) | Blue-grey | ~0.1% |
The lesson hides in the last column. The overwhelming majority of diamonds are Type Ia. The Type IIa stones — free of nitrogen, often exceptionally limpid — are rare enough to set auction records, and most fancy-coloured diamonds belong to the type II families.
The four, in order of effect
- Cut governs how light enters, bounces and leaves — the brilliance, fire and scintillation you actually see. It is the most human of the four: the only C made entirely by hand.
- Colour measures how far a stone departs from colourless, on the D-to-Z scale.
- Clarity records the inclusions formed as the crystal grew, read at ten-times magnification.
- Carat is weight, not size — and the most visible lever on price.
Each section of this guide takes one of the four in turn.
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