About Gem Index
Gem Index is an independent, free tool that estimates the indicative wholesale trade value of coloured gemstones from the characteristics you can see and measure.
Why we built it
Coloured-stone pricing is famously opaque. Unlike diamonds, coloured gems have no single published price grid — value swings with colour, origin, treatment, clarity, cut and size, and those factors interact differently for every species. A fine unheated Kashmir sapphire and a heated commercial one can differ in price by two orders of magnitude while looking superficially similar. That opacity is hard on hobbyists, inheritors, small dealers and buyers who simply want a sanity check before they trade.
Gem Index exists to give a transparent, repeatable first estimate. You describe the stone; the engine applies a species-specific model and returns an indicative per-carat trade range, along with a breakdown of how each factor moved the number. It will never replace an expert appraisal, but it turns "I have no idea" into "I have a defensible ballpark."
How the estimate is produced
Each of the 78+ supported varieties has a market-seeded base price per carat. That base is then adjusted multiplicatively for colour (tone and saturation), clarity and transparency, cut and finish, treatment, origin, certification and size. The model is calibrated for transparent faceted coloured stones. Our methodology page explains the logic in detail.
What Gem Index is not
- Not an appraisal. The figures are indicative trade estimates, not a certified valuation for insurance, tax, estate or resale.
- Not a lab report. The engine trusts the attributes you enter. It cannot detect synthetics, undisclosed treatments or misidentified species — only an accredited gem laboratory can.
- Not a marketplace. We do not buy, sell, broker or authenticate stones, and we take no commission on any transaction.
Independence
Gem Index is a personal project and is not affiliated with any gem laboratory, auction house or dealer. The site is supported by advertising, which never influences the pricing model. Constants in the engine are refined over time as more cleared-sale data becomes available.
Contact
Questions, corrections or calibration data are genuinely welcome — see the contact page. Feedback from working dealers and gemmologists is the main way the model improves.