Here is the
post referring to gem grade. I'd contact them to see what you can learn about this option. One thing I have noticed in my own pearls is that the gem grade pearls really seem to leap off my neck they are so lustrous. And as much as 'bigger is better' in a lot of ways, the gem grade pearls can have a serious impact at a slightly smaller size. It's a balance game between size and quality, but I think that quality is almost always worth it in the long run.
Keep us posted on what you decide!