sleek,
The dark coloration runs the length of the stave like heartwood. On the finished bow, I removed all but a few traces of it along the limb bellies. Where it remained it left some nice "flames" in the fades and runs the length of the handle. The only other piece with the same color out of the 8 stave I cut is the one I'm building for the trade. I know how it formed, so there may be some more lurking in the hazel grave where I cut from. The original tree had a "Y" right near the ground. The other branch had died and fallen leaving a big hollow knot hole. Over time it filled with organics which then turned to soil. There was actually a small maple sprout growing out of it last time I walked by. The fungus in that soil entered the stave at that "y and ran up the center. Only the first 1/2 foot of the stave had a rotten center. The rest of the way up was that wonderful chocolate colored stain. I'll be up near that patch of hazel every day this week. I'll keep my eyes peeled for a similar situation in another tree. If I find one, I'll be glad to set up a trade. Let's just hope I can find a straight one.
I'll try to get a full write up on that bow posted in it's own thread in the next couple days. I had intended to do so anyway seeing how it's probably one of the prettiest bows I've made thus far.
Thanks for your interest.