Thanks for the in depth answers. The only thing I'm now unsure of is the difference between corrosion and toning. For example, the coin I put in my comment to you yesterday...

...had a high bid about 15% less than the other coins at the same grade.

So, is this particular coin toned or crud/corrosion? The slab says "toned".

I have some follow-up questions if you feel like it.

What is toning? Like, what is the stuff on the coin? How does it get there?
I've seen what looks like fake toning, is that something different? Are there different kinds of toning?
What is the difference between crud/corrosion and toning, chemically speaking.

Thanks again professor, I appreciate the answer.
!PIMP