There is a pretty decent article on graphite on wikipedia.
Graphite - Wikipedia, the free encyclopedia
I was surprised that in the US, most graphite is synthetic, and most internationally sourced is from natural sources. It didn't surprise me that the US supply is mostly synthetic, I just didn't realize it was a natural product.
The range of variations was also a surprise.
The reason I looked at graphite, is that I need a reasonably pure carbon source for a project, and wondered if scap graphite from machining was a viable source for it. The answer seems to be maybe.