Granted, I have not made many pens yet, but the worst so far was a chunk of pink ivory. The blank had a nice crack in it that I figured I could work around. Well, I did for a bit as I had pretty much filled the crack really well with CA.
Then when turning it the blank pretty much exploded. It just blew apart right on the lathe. After a lot of griping and cussing and stomping my feet, I glued it back up and put it back on the lathe. No more chunks of wood flying around, thankfully, but then the gouges had to keep being resharpened. After two and a half hours of this, I had the first of the two peices done.
So, I did the second part of the pen. Again, the chunk blew apart and again I fixed it. Another two hours of this and the pen was ready to be sanded.
I must say, all the trouble was worth it. It came out really well - the wood around the cracks turned a much darker color so it makes them stand out. I don't know how to describe it really, other than to say it almost looks like a chunk of marble.
I made the pen for a friend of mine and she seemed to really like it.
But, for now, I refuse to work with pink ivory for a while.