Carat weight
One Carat is equal to 200mg.
The weight of the diamond is measured in carats to 2 decimal places using specially built diamond scales. A diamond of 0.50ct is called a half carat diamond, 0.75ct is a three-quarter carat diamond, etc.
A diamonds weight is commonly referred to as the 'size' of the diamond. In the jewellery trade, the terms weight and size are interchangeable because a well proportioned diamond should fall within a certain diameter range for its weight.
To put into context the rarity of larger diamonds, a one carat (1.00ct) diamond of the same quality as a $2000 half carat (0.50ct)diamond may sell for as much as $9000. While only twice the weight it is 4.5 times the price.