Moissanite Grading & Size Chart
Everything below is calculated, not copied: the carat-to-millimetre maths uses the GIA round-brilliant formula scaled by each stone’s density, and the colour and clarity scales follow the GIA system the moissanite trade has adopted. Use it to work out what size stone you are actually buying.
Carat → millimetre calculator Round brilliant
Nearly every carat/mm chart online is calculated for diamond. Moissanite is less dense, so the same millimetre size weighs about 9% less. Enter either value below to convert correctly for each stone.
These figures are for round brilliant cut stones only. Fancy shapes — oval, princess, emerald, pear — spread their weight differently, so an oval of the same carat looks larger face-up than a round. Use this as a round-cut reference, not a universal chart.
| Stone | Carat | Diameter | Relative size |
|---|
Round brilliant, 61.2% depth ratio, calculated with the GIA formula scaled by each stone’s density (moissanite 3.21, diamond 3.52, cubic zirconia 5.70 g/cm³). Real stones vary with cut proportions — treat as guidance, not a substitute for weighing.
Colour grades (D–K)
Moissanite uses the GIA colour scale. Most FITINY pieces are D colour — the top of the colourless band.
| Grade | Band | Tint | What it means |
|---|---|---|---|
| D | Colourless | none | Highest grade. No detectable colour under 10x magnification against a white background. |
| E | Colourless | none | Minute traces of colour, detectable only by a trained grader. |
| F | Colourless | none | Slight colour detectable by an expert; still classed as colourless. |
| G | Near colourless | very faint warm | Colour noticeable only when compared side by side with D-F. |
| H | Near colourless | faint warm | Appears colourless face-up in most settings. |
| I | Near colourless | faint warm | Slight warmth visible face-up to a careful eye. |
| J | Near colourless | noticeable warm | Warmth visible face-up, more so in larger stones. |
| K | Faint colour | visible warm | Faint yellow visible to the unaided eye; often paired with yellow gold. |
Clarity grades
| Grade | Name | Under 10× | Eye clean | In moissanite |
|---|---|---|---|---|
| FL | Flawless | No inclusions or blemishes | Yes | — |
| IF | Internally Flawless | No inclusions; minor surface blemishes only | Yes | — |
| VVS1 | Very Very Slightly Included 1 | Inclusions extremely difficult to see at 10x | Yes | Common |
| VVS2 | Very Very Slightly Included 2 | Inclusions very difficult to see at 10x | Yes | Common |
| VS1 | Very Slightly Included 1 | Inclusions difficult to see at 10x | Yes | — |
| VS2 | Very Slightly Included 2 | Inclusions somewhat easy to see at 10x | Yes | — |
| SI1 | Slightly Included 1 | Inclusions noticeable at 10x | Usually | — |
| SI2 | Slightly Included 2 | Inclusions easily noticeable at 10x | Sometimes | — |
Moissanite vs diamond vs cubic zirconia
| Stone | Refractive index | Dispersion (fire) | Mohs hardness | Density g/cm³ |
|---|---|---|---|---|
| Moissanite | 2.65–2.69 | 0.104 | 9.25 | 3.21 |
| Diamond | 2.417–2.419 | 0.044 | 10 | 3.52 |
| Cubic zirconia | 2.15–2.18 | 0.060 | 8.25 | 5.70 |
Dispersion is what the eye reads as “fire”. Moissanite’s 0.104 is more than twice diamond’s 0.044 — which is why it flashes more colour.
Keep reading
Where these numbers come from
Nothing on this page was copied from another chart. The colour and clarity scales follow the GIA system the moissanite trade uses, and the carat/millimetre figures are computed from GIA’s round-brilliant formula scaled by each stone’s density. You can read the data and the maths, or run it yourself:
github.com/FITINY/moissanite-tools — open source (MIT), includes an MCP server so an AI assistant can answer these questions directly.