This is simply the measure of weight for stones. Something helpful for you in this area is to know that diamond prices are not linear to size. Or in other words a 1ct diamond is not twice the price of a ½ carat diamond. As the size increases the rarity of a nice clear good coloured stone increases, pushing the price respectively. Diamonds are often in price brackets also. You may find a 49pt to be a little less than a true 50pt an so on.