Monty comes closest with his guesses. Actually, it is "fost" wood (found on side of tree). It is part of a tree fungi that grows on the side of dead (or dying) trees. They can be found in a variety of sizes and colorations. I've only made a couple pens as they are very porous and built like a hard sponge so they require a lot of stabilizing with CA (I'm not a caster - yet) as you turn. But the couple pens I've made have neat wood-like grain and lots of different color/depth patterns. If they were not such a pain, I'd do them more often.
By the way - there are some great guesses here. And as Ronald Reagan might have said, "Well, if you've enjoyed playing the game half as much as I've enjoyed offering it, then..... I've enjoyed it twice as much as you have." :biggrin: