The ‘Fourth C’
The term carat refers to the unit of weight used to measure all gemstones. It originates from the seed of the carob tree: remarkably uniform in weight and used by merchants to balance scales to weigh gems.
A carat is 1/5th of a gram and is further divided into ‘points’ – 100 points = 1 carat, 50 points (or 0.50 ct) is a half-carat diamond.
The weight of a diamond relates very closely to its size (diameter) but variations can occur if the cutter departs from ideal proportions.
Thus a well proportioned, one-carat brilliant cut diamond should be close to 6.4 mm in diameter. Due to its heavier specific gravity, a round sapphire of the same diameter may weigh around 1.15 ct, while an amethyst of similar size should weigh approximately 0.75 ct.