Gemstone Price Per Carat Calculator
Calculate gemstone price per carat and total cost by quality grade.
Returns retail, wholesale, and resale estimates for diamonds, rubies, and sapphires.
Gemstone pricing follows a unique pattern: the price per carat increases exponentially with size, not linearly. This is because larger stones of good quality are exponentially rarer than smaller ones.
Basic Price Formula
Total Price = Price Per Carat × Carat Weight
But the per-carat price itself depends on size. The relationship is usually written as:
Total Price = Base × (Weight)^n
Where n runs about 1.7 to 1.9 for precious stones. That exponent applies to the total price, not the per-carat price, and the distinction trips people up constantly. Since total = per-carat × weight, the per-carat figure scales at n minus 1, so roughly Weight^0.8. A 2-carat stone therefore costs about 3.5 times a 1-carat stone of the same quality, of which a factor of 2 is simply having twice the weight and the remaining 1.75 is the size premium.
Price Per Carat Reference (approximate retail USD)
These are order-of-magnitude reference points for a 1-carat stone, gathered from retail listings and revised infrequently. Gem pricing moves with fashion, treatment disclosure and origin, and any individual stone can sit well outside its row. Use them to sanity-check a quote, never as a valuation.
| Gemstone | Low Quality | Medium Quality | High Quality | Exceptional |
|---|---|---|---|---|
| Diamond (1 ct) | $2,000 | $5,000 | $12,000 | $25,000+ |
| Ruby (1 ct) | $500 | $2,500 | $8,000 | $20,000+ |
| Sapphire (1 ct) | $400 | $1,500 | $5,000 | $15,000+ |
| Emerald (1 ct) | $300 | $1,500 | $5,000 | $12,000+ |
| Tanzanite (1 ct) | $100 | $400 | $800 | $1,200 |
| Aquamarine (1 ct) | $50 | $200 | $500 | $1,000 |
| Amethyst (1 ct) | $5 | $15 | $30 | $50 |
| Garnet (1 ct) | $10 | $50 | $200 | $500 |
| Opal (1 ct) | $20 | $100 | $500 | $3,000+ |
| Tourmaline (1 ct) | $50 | $200 | $500 | $2,000+ |
Size Premium Factor
Price per carat rises with weight. Against the 1-carat reference above, the multiplier the calculator applies is Weight^0.8:
| Weight | Per-carat multiplier | What the whole stone costs, relative to 1 ct |
|---|---|---|
| 0.25 ct | 0.33x | 0.08x |
| 0.50 ct | 0.57x | 0.29x |
| 1.00 ct | 1.00x | 1.00x |
| 1.50 ct | 1.38x | 2.07x |
| 2.00 ct | 1.74x | 3.48x |
| 3.00 ct | 2.41x | 7.22x |
| 5.00 ct | 3.62x | 18.12x |
The right-hand column is where the money is. Going from one carat to two roughly triples the price; going to five multiplies it eighteen-fold. This is also why cutters will sacrifice proportions to keep a stone above a round number: a 0.98 ct diamond and a 1.02 ct diamond are separated by far more than 4% of price, because the market prices the label as much as the weight.
Above about 5 carats the curve steepens further for the precious stones, since supply of clean large material effectively stops. Treat anything past that as a guide only.
Worked Example
A 1.8 carat medium-quality sapphire:
- Reference per-carat at 1 carat (medium): $1,500
- Size factor at 1.8 ct: 1.8^0.8 = 1.60
- Expected per-carat: $1,500 × 1.60 = $2,401
- Expected total: 1.8 × $2,401 = $4,321
Pay $6,000 for that stone and you are 39% above the reference, which is not automatically a bad deal. It may be a Ceylon origin, an unheated stone, or simply a better color than “medium” describes. It does mean you should ask which.
Reading the comparison
The calculator works backwards from what you actually paid. Enter the total price and the weight, and it reports your price per carat against the size-adjusted reference for that stone and grade. Anything within about 10% either way is the reference doing its job. A large gap in either direction is a question, not an answer:
- Far below reference: possible synthetic, undisclosed treatment, or a grade one step lower than you were told. Get it certified.
- Far above reference: often legitimate. Origin, absence of heat treatment, and certification all carry real premiums that no per-carat table captures.
The “Four Cs” (Cut, Color, Clarity, Carat) were built for diamonds, but the same factors move colored stone prices too. Origin matters enormously: Burmese rubies, Kashmir sapphires and Colombian emeralds command multiples over identical-looking stones from elsewhere, and only a laboratory report will tell you which you have.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.