Carat

One carat equals exactly 200 milligrams, or 0.2 grams. That standard was adopted internationally in 1907 and hasn't changed since. Carats subdivide into 100 points. A 0.50ct stone is a "50-pointer." A 0.25ct stone is a "quarter carat." Dealers use points constantly in trade talk. Carat measures weight, not size. Two stones at the same carat weight can look different on the finger depending on cut proportions, depth, and shape. A 1.00ct round brilliant typically measures about 6.5mm across. A 1.00ct oval might measure 8mm x 6mm and appear larger. A 1.00ct emerald cut looks smaller face-up because more weight sits in the pavilion depth. Specific gravity matters too. A 1.00ct sapphire (density ~4.0) is physically smaller than a 1.00ct diamond (density ~3.52) because sapphire packs more mass into less volume. Magic weights create price cliffs in the diamond trade. At 0.50ct, 1.00ct, 1.50ct, and 2.00ct, per-carat prices jump 15-40%. A 0.98ct diamond might cost $4,200. The identical stone at 1.01ct costs $5,800. Same look on the finger. Different number on the certificate. Smart buyers target just-under weights: 0.90ct instead of 1.00ct, 1.40ct instead of 1.50ct. Carat vs karat trips up almost everyone. Carat (ct) is gemstone weight. Karat (k or kt) is gold purity: 14k, 18k, 24k. Different words, different systems, similar spelling. I've watched a client ask for a "24 carat gold ring with a 1 karat diamond" and mean the exact opposite.