Free gemstone calculators and charts
Estimate carat weight from size, convert carats to grams, work out cutting yield, profit and parcel costs, and look up hardness, density and birthstones. No sign-up.
Calculators
Carat weight from size
Estimate a cut gemstone's carat weight from its length, width and depth in millimetres. Choose the gem and shape. Includes a round stone mm-to-carat chart.
Open toolYield and cost per carat
Work out the yield, weight loss and cost per finished carat when you cut rough into a polished stone.
Open toolPrice per carat and total
Convert between price per carat and total price, and add a markup to find your selling price.
Open toolProfit, margin and markup
Work out profit, margin, markup and the selling price you need for a target margin on a gemstone sale.
Open toolSplit a parcel's cost per stone
Enter a parcel's total cost and the weight of each stone to split the cost across the stones by weight. Shows cost per stone and per carat.
Open toolReference charts
Mohs hardness of gems
Mohs hardness of popular gemstones from diamond (10) to pearl, sorted hardest to softest, with what it means for wear and cutting.
Open toolSpecific gravity and RI
Specific gravity (density), refractive index and hardness of 19 popular gemstones in one table. Handy for identification and weight estimates.
Open toolBirthstone by month
The modern birthstone for every month, from garnet in January to tanzanite, turquoise and zircon in December.
Open toolOutgrown the calculators?
GemColony keeps every stone's weight, cost, price and history in one place, splits parcels, tracks cutting yield and memos, and reports your margins automatically.
Questions about the tools
Are these gemstone tools free?
Yes. They run in your browser, need no account and store nothing.
How do I convert carats to grams?
Multiply carats by 0.2. One carat is exactly 0.2 grams, so 5 carats make 1 gram. The carat converter does it for you.
How can I estimate a gemstone's carat weight from its size?
Use the gemstone weight calculator: enter length, width and depth in millimetres, pick the gem and cut, and it estimates the weight, usually within about 10%.
How do I work out cutting yield?
Divide the finished weight by the rough weight and multiply by 100. The cutting yield calculator also shows cost per finished carat.
Who are these tools for?
Gem dealers, cutters, jewellers and buyers. They do arithmetic and show reference values; they do not identify stones or give valuations.