Carat
/KAIR-ut/
Definition
A unit of weight used to measure gemstones, where one carat equals 200 milligrams (0.2 grams). Not to be confused with 'karat,' which measures gold purity.
The carat is the universal standard for gemstone weight, adopted internationally in 1907. One carat (ct) equals 200 milligrams, and carats are subdivided into 100 'points' — so a 0.50ct diamond is also called a '50-pointer.' Crucially, carat measures weight, not size: two stones of the same carat weight can have different dimensions depending on their cut proportions, depth, and specific gravity. A 1-carat sapphire (SG ~4.0) will be visually smaller than a 1-carat diamond (SG ~3.52) because sapphire is denser. In the diamond trade, prices per carat increase at 'magic numbers' — 0.50ct, 1.00ct, 1.50ct, 2.00ct — creating significant price jumps at these benchmarks.
Usage in the Trade
Carat weight is used universally in the gemstone and jewelry trade for pricing, grading, and describing gemstones. It appears on grading reports, price lists, and invoices.
Related Resources
Related Terms
Related Gemstones
Diamond
The hardest natural substance and the benchmark of the gemstone world.
Emerald
The green jewel of royalty, prized for its saturated color above all else.
Ruby
The king of colored gemstones, valued for its vivid red brilliance.
Sapphire
Corundum in every color except red — the most versatile precious stone.