{
  "markdown": "# moissanite-tools\n\n[![smithery badge](https://smithery.ai/badge/fitiny/moissanite-tools)](https://smithery.ai/servers/fitiny/moissanite-tools)\n\n![Moissanite grading and size chart](assets/og-moissanite-grading.png)\n\nGrading reference and carat↔millimetre conversion for **moissanite** — calculated, not copied.\n\n## Why this exists\n\nPractically every carat/mm chart on the web is calculated for **diamond**. Moissanite has a\ndifferent density (3.21 vs 3.52 g/cm³), so applying a diamond chart to moissanite is wrong by\nroughly 9%:\n\n| Diameter | Diamond | Moissanite | Cubic zirconia |\n|---:|---:|---:|---:|\n| 4.0 mm | 0.24 ct | 0.22 ct | 0.39 ct |\n| 5.0 mm | 0.47 ct | 0.43 ct | 0.76 ct |\n| 6.5 mm | 1.03 ct | 0.94 ct | 1.66 ct |\n| 8.0 mm | 1.91 ct | 1.74 ct | 3.09 ct |\n\nThis is why moissanite is usually sold by millimetre rather than by carat — the same \"1 carat\"\nlooks like a different size depending on the stone.\n\n## What's in here\n\n```\ndata/grading.json    GIA colour (D–K) and clarity (FL–SI) scales, plus published\n                     optical constants for moissanite / diamond / cubic zirconia\ndata/sizing.json     ISO 8653 ring sizes (US/UK/EU/mm), necklace lengths, stud sizes\nsrc/conversion.js    carat ↔ mm for round brilliant cut, density-corrected\nsrc/sizing.js        ring size conversion, necklace length and stud appearance lookups\nmcp/server.js        Model Context Protocol server exposing all of the above\n```\n\n## Ring sizes\n\nCross-border ring buying goes wrong because the systems are unrelated: **a US 6 is a UK M**,\nnot a UK 6. EU sizes under ISO 8653 are simply the inner circumference in millimetres.\n\n| US | UK | EU | Inner Ø |\n|---:|:--|---:|---:|\n| 5 | J | 49.3 | 15.7 mm |\n| 6 | M | 51.9 | 16.5 mm |\n| 7 | O | 54.4 | 17.3 mm |\n| 8 | Q | 57.0 | 18.1 mm |\n| 9 | R½ | 59.5 | 18.9 mm |\n\nSizes that aren't on the chart return the nearest listed size flagged `exact: false`,\nrather than inventing a value.\n\n## Usage\n\n```js\nimport { caratFromMm, mmFromCarat, compareAtWeight } from './src/conversion.js';\n\nmmFromCarat(1, 'moissanite');   // 6.65 mm\nmmFromCarat(1, 'diamond');      // 6.45 mm\ncaratFromMm(6.5, 'moissanite'); // 0.94 ct\n\ncompareAtWeight(1);\n// { carat: 1, moissanite_mm: 6.65, diamond_mm: 6.45, cz_mm: 5.49 }\n```\n\n### MCP server\n\nTwo ways to run it.\n\n**Hosted — nothing to install.** Point any MCP client at:\n\n```\nhttps://mcp.fitiny.com/mcp\n```\n\nListed in the official MCP Registry as `com.fitiny/moissanite-tools`.\n\n**Local — stdio:**\n\n```bash\nnode mcp/server.js\n```\n\nExposes six tools:\n\n| Tool | Answers |\n|---|---|\n| `carat_to_mm` | How wide is a 1 ct moissanite? |\n| `mm_to_carat` | What does a 6.5 mm stone weigh in each material? |\n| `grading_scale` | What does VVS1 / D colour actually mean? |\n| `ring_size_convert` | What is a UK M in US sizing? |\n| `necklace_length` | Where does an 18\" necklace sit? |\n| `stud_appearance` | How big does a 6 mm stud read on the ear? |\n\n## The maths\n\nGIA's round-brilliant formula, scaled by relative density:\n\n```\nweight_ct = diameter_mm² × depth_mm × 0.0061 × (ρ_stone / ρ_diamond)\ndepth_mm  = diameter_mm × 0.612        (average depth ratio, well-cut round)\n```\n\nAccuracy against published diamond charts: **1.4% mean, 3.3% max**. Real stones vary with cut\nproportions — treat the output as guidance, not as a substitute for weighing a stone.\n\n**Round brilliant only.** Fancy shapes (oval, princess, emerald, pear) distribute weight\ndifferently and need shape-specific factors that aren't in here.\n\n## On the data\n\nColour and clarity scales follow the GIA diamond grading system, which the moissanite trade has\nadopted. The optical constants (refractive index, dispersion, Mohs hardness, density) are\npublished gemmological values. **Nothing here was scraped from another site** — the conversion\nfigures are computed from the formula above, which is why they can be released under MIT.\n\nThis is a descriptive reference, not a certification. A graded stone comes with a report from a\ngemmological laboratory; this repo just explains what those grades mean.\n\n## Live tool\n\nAn interactive version runs at\n[fitiny.com/pages/moissanite-grading](https://fitiny.com/pages/moissanite-grading).\n\n## Licence\n\nMIT — see [LICENSE](LICENSE). Maintained by [FITINY Jewelry](https://fitiny.com).\n",
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