The weight of a gemstone is measured in carats. One carat is equivalent to 200 milligrams (0.20 grams) and is divided into 100 points for precision. A sapphire weighing half of a carat may be referred to as 0.50 carats.
Carat weight and size are not the same and must not be confused for two reasons:
1. Two gemstones of the same type, carat weight, and shape may appear different due to differing cuts. The deeper cut stone is likely to appear smaller than the other.
2. Gemstones have differing densities (mass per unit volume). Sapphire, for instance, is denser than a diamond, meaning a one-carat sapphire would appear smaller than a one-carat diamond.
The best way to visualize the size of a gemstone is by its dimensions, the top diameter in particular.