Could be one of several things...Koa or Black Acacia (a koa relative), or even mesquite from a curly part (root) of the tree. I have some that looks just like that.
Oh yeah, I've seen some cuts of Monkey Pod look just like that too.
I just re-read your post a bit better...if it was extremely light, the possibility of it being Monkey pod is much higher as Koa is a fairly dense wood. Monkey Pod is very light and not dense at all. Did it seem real dusty when you turned it?
BYW- those are nice pens.
Lou, I am no expert on wood...might be able to recognize a few but thats about it.[:I]