Carat
Carat is the first thing we think of when talking about a Diamond. Carat is what the diamond weighs when put on a scale. This number (.50ct, 1ct, 3ct etc) is how most people describe the size of their stone. What is less known about carat is the dimension of the stone. For example a 1 carat diamond should measure approximately 6.5mm in diameter. Now since all the 4 C's realte to each other, we jump back to cut, which explains why one 1ct diamond may measure 6.5mm and the other 1ct diamond may measure 6.0mm. A well cut diamond, that is within its correct proportions will weight and look like its weight should, a lesser cut stone with variying measurements will still weigh the same but will look slightly different loose, and in the setting you choose.
Pro Tip: Try to not let the weight of the stone sway your decision, buy a diamond based on the look of the stone, not what is weighs on the scale.