In My experience, Darren,
dealers will call the grade of pearl anything that sounds good at the time, however, when makers buy it from the suppliers, it is graded in a way that's more meaningful, but by the time the dealer or maker sells the knife, it's anybody's guess what the grade actually was. The only way to know is to see the pearl with your eyes and decide for yourself how nice it is. I love "exhibition grade" pearl, because this means, the knife is now suitable to be looked at.
One way to try to judge pearl, short of seeing it in person, is by the maker, some makers are known for the quality of their pearl and keep it consistantly high, same with stag and ivory. For example, I have never seen a Pease folder with less that top of the line pearl, nor a Dake, for stag, Pease is also one of the best, and from my recent, though limited experience with Don Hanson, I would say his pearl, ivory and stag are exceptional.
Here is some pearl on a Pease folder of mine, it remains the best white pearl I have yet seen.