Carat

A carat is the trade's unit of weight: one fifth of a gram, divided into one hundred points. A dealer will call a 1.52 ct stone "one fifty-two". Weight is not size. Denser species pack more carats into fewer millimetres, so a one carat sapphire faces up smaller than a one carat emerald, and a one carat spinel sits between the two. Buy with a millimetre gauge in your head as well as a scale, since the setting will show spread, not weight.

Prices step at threshold weights. A stone just over one carat trades in a different bracket from one just under, and the same happens at two, three and five, which is why so many stones sit just above a round number and why recutting a stone below one is a decision taken slowly. Cutters know the steps and leave weight on the stone to reach them, sometimes at the cost of a window or a fat pavilion. Those extra points can cost more than they add.

On a lab report

Every report states weight to two decimal places, with measurements in millimetres in the same block: length, width and depth for a faceted stone. Read the two together. A stone that is heavy for its face-up measurements is carrying its weight in the pavilion; one that is light for its spread has been cut shallow. Weigh the stone yourself against the report before paying. A gap of a few points is a certificate mismatch until it is explained, and the invoice should carry the same figure as the report to the second decimal.

Parcels and calibrated goods are priced per carat, single fine stones by the piece with a per-carat rate quoted alongside. Ask for the weight and the millimetre spread side by side, and price the face-up size you will actually see rather than the number on the paper.