Even though most of us think of a carat as a measure of size, it’s actually a measure of weight. Historically, the carob seed was used on balance scales to determine weight for gemstones, and this is where our modern carat came from.

Carob pods and seeds

A carat weighs 1/5 gram; further, a carat is divided into one hundred points.  So, a 50 point diamond is the same as a half carat, and is usually written as 0.50 carat.

Does a one carat diamond look twice as large as a 1/2 carat diamond? It doesn’t work that way.  Remember, carat is a measure of weight, and weight is added all over the diamond (volume) instead of what we see face-up (diameter).

A typical one carat round brilliant diamond is 6.5 millimeters in diameter. A half-carat round brilliant measures 5.1 millimeters in diameter. So, a round brilliant diamond with twice the diameter of a one carat round brilliant would weigh a little over eight carats.