Is that Ancient Kauri?
As the tree endures weather phenomenon that cause the tree to sway back and forth, the outer layers can "slip" against each other as they form when they are still soft ... this slippage results in a rippling effect along the skin of the tree as it grows, and when that layer becomes part of the wood of the tree it "freezes" in that shape. Subsequent layers forming on top will only add to this phenomenon, though the exact same species of tree living in the very same valley or mountain may not end up with this "curly" pattern you have there just because the wind patterns were different or temperatures or humidity allowed the outer layer of the tree to straighten out, or it just never bent in the wind far enough that the slippage would occur. (Of course I could be wrong about this ... I'm sure someone will come along and let us know!)
It seems you have a piece of Ancient Kauri that exhibits "whitebait" as well as quite a bit of curl. Not many wood species have this level of chatoyance, though I've seen something approaching it in certain species of wood like Koa.
It's supposed to be caused not only by the wavy formation of the wood itself, but by the oils present in the wood causing light to refract in the very first millimeter of the wood.
In the case of Koa, the wood itself is quite hard and a little oily, but not much. In the case of Ancient Kauri, the oils are from the bog that had captured the tree when it fell some 50,000 years ago and preserved it away from oxygen for all this time.
If you check out the "Pretty Wood Pen" voting thread, you'll see quite a number of pens that have a similar chatoyant effect in the wood. Once you discover which species will have that kind of effect, remember it ... it's beneficial to achieve that effect by stepping up several levels in your final sanding efforts before applying your finish.
Ancient Kauri is recommended for final sanding (on bare wood) at 1200 - 1500 grit. It's gorgeous stuff!
