Carat (Ct) weight.

Diamonds are weighed in carats (ct). 1 carat equals 0.2gm and there are 100 points in a carat. Therefor 0.50ct is equals to 50 points which is half a carat. All diamonds are weighed in this way throughout the world unlike other gemstones which are often weighed in grams.

Attached is a rough guide on approximate stone weight by measurement of the length and depth of the polished stone to give you a visual idea.