Originally posted by BWD: It can be the wood, or the finish, or sometimes both.
Originally posted by Jeremy: Many woods darken with exposure to sun and oxygen. I have some pieces of bocote and cocobolo in the shop that are nearly black on the surface There are some woods that get lighter with exposure. Black Walnut comes to mind and I believe is probably the most extreme. I think Wenge as well. Paduak loses its red.