Field report · 737
The carat / gram distinction: when each is used
Why faceted stones are weighed in carats and specimens in grams — and how to convert between them.
Two units, two markets. Carat for faceted stones priced per-carat, grams (or kilograms) for everything else. Here's the practical guide.
What's a carat?
A carat (ct) is exactly 200 milligrams, or 0.2 grams. The word comes from the carob seed, which was historically used as a standardized small-weight reference (though carob seeds vary more than the legend suggests — the 200mg standard was set by international agreement in 1907).
When carat is used
- Faceted gemstones for jewelry — diamonds, sapphires, rubies, emeralds, tanzanite, all the colored stones above $50/ct
- Pearls (sometimes) — though grain (1 grain = 0.25 carat = 50mg) is also used in pearl trade
Why carat? Because faceted stones are priced per-unit-weight, and the per-carat price varies dramatically — a 1ct stone might be $500/ct while a 5ct stone of the same quality is $2,000/ct. The carat unit captures meaningful price differences in small increments.
When grams (or kilograms) are used
- Cabochons and beads — often grams
- Crystal specimens — grams for hand-sized, kilograms for larger
- Rough material — grams or kilograms (or pounds, in some markets)
- Tumbled stones — grams or pounds
- Lapidary material — kilograms for slabs and chunks
Why grams? Because larger pieces aren't priced per-unit-weight; they're priced as objects with a quoted weight for reference. The reference unit just needs to be sensible at the size involved.
Practical conversions
- 1 carat = 0.2 grams = 200 milligrams
- 1 gram = 5 carats
- 1 kilogram = 5,000 carats (= 1 kilo-carat, abbreviated kct, sometimes used for very large stones)
- 1 ounce (avdp.) ≈ 141.7 carats
Common size reference points
| Weight | Typical visual size |
|---|---|
| 0.25 ct | 3mm round (small accent stone) |
| 0.5 ct | 5mm round |
| 1 ct | 6-7mm round / center stone |
| 2 ct | 8mm round |
| 5 ct | 10mm round (oval ring stone) |
| 10 ct | 2g cabochon (palm of hand) |
| 50 ct | 10g specimen |
| 500 ct (100g) | palm-sized specimen |
| 5,000 ct (1kg) | cabinet-sized specimen |
Pricing patterns
For per-carat priced stones, the unit price almost always increases with stone size — a 2ct stone of the same quality as a 1ct stone may cost 4-8x more per carat, not just per stone. This is the "carat premium" curve and it accelerates above 5ct.
For gram-priced specimens, pricing is per-piece with weight as a reference. A 50g amethyst point might cost $80; a 100g amethyst point might cost $200 — not double, because the larger piece is more dramatic.