The carat weight is quite simple and refers to the diamonds weight. There are a few tricks about carat weight when buying a diamond. One trick when purchasing a diamond is to not focus too much on the actual weight, as when you hit certain carat weights the price will jump. For example, a 1.00 ct G-SI1 diamond and a .90 ct G-SI1 diamond will effectively look the same but the 1.00 ct diamond will cost $1,000 or so more!
Another trick is again to not get lost in the actual carat weight but focus on the mm measurements. In the industry we used the term “spready” which means a diamond looks bigger than the actual carat weight. You can have a 1.00 ct diamond that is “spready”, giving the appearance of a 1.25 ct stone!