Just because someone has a carat ring doesn't mean it's a great diamond. Clarity Diamonds spend forever in the ground. The rocks, dirt, and other nasty stuff around them can scar them. Some of these scars are visible to the naked eye, but others can only be seen under a microscope. You must pay attention to the clarity of a diamond when purchasing it.
Clarity is referred to with weird scientific labels.
You need to become familiar with them. The table below points out what the labels mean, such as IF, VVS, SI, etc. Until this point you probably thought all diamonds were clear, or white crystal looking things. Look closely at the color of the diamonds you're considering purchasing. Most girls want it as white as possible, but a little color is not noticeable.