Carat

"A larger diameter for a given weight will make a diamond appear bigger, but it is not always better. It will often appear lifeless and be lacking in sparkle."

Carat is the internationally recognised measurement of the literal weight of the stone. The GIA makes it clear that one  carat converts to 0.2 grams, which is the very same weight as a paper clip!

It makes sense that the larger the carat, the higher the price you can expect to pay. No two stones are exactly the same, which means that carat should only be seen as a rough guideline - it only establishes the weight of the stone, not the physical size. After all, you can have five oval diamonds, all weighing in at 2.00 carats, and all of them ranked as the same colour, clarity, and cut, but they still feature slightly different sizes and shapes.

The carat system uses 100 points, meaning a 50-point diamond weighs in at 0.50 carats. A stone with a certain weight may appear larger than the carat indicates because of its dimensions (presented in millimeters). For instance, you might identify a stone that weighs 2.00 carats but appears more like a 2.20 carat stone. 

In this case, you have an opportunity to purchase a diamond that looks larger, without the relatively higher price that you would associate with a greater weight.