Next time you get a chance, pop into your local jewelry store and take a look at the gems. Notice anything about them? They're all tiny, on the order of the size of a pinhead. If you pried every stone in the place loose from its setting and piled them up, you would be arrested the entire heap would probably fit in a soup pot. In Real Life, gem-quality stones larger than a person's fingernail are so rare that they are almost never for sale on the open market. This doesn't apply in fiction, where a single gem can easily be the size of your fist... head... torso... you get the idea. It's also not very uncommon if gems that are mined are already cut.