Carat
A unit of gemstone weight, equal to 0.2 grams. Nothing to do with karat, which measures gold purity.
One carat is 200 milligrams, divided into 100 points. A 50-point stone is half a carat.
Carat is mass, not size. Two one-carat stones can look noticeably different depending on how they are cut, because a deep cut hides weight below the surface.
Pearls are the exception. They are sold by millimetre diameter rather than carat, though the old Japanese unit momme still appears in wholesale.
When you are buying
When an Indian listing says carat about metal, it almost always means karat. Ask which one is meant before you compare two prices, because the words are used interchangeably and the numbers are not.
Karat
The purity of gold, measured in twenty-fourths. 24K is pure gold; 22K is 22 parts gold to 2 parts other metal.
Momme
A Japanese unit of weight used in the pearl trade, equal to 3.75 grams. Used wholesale, rarely at retail.
Pearl Size
The diameter of a pearl in millimetres, measured across its widest point. Price rises steeply, not evenly, with size.
Cubic Zirconia
A hard, brilliant lab-made stone used as a diamond substitute. Called American diamond throughout the Indian trade.