I also feel that the pump gun Ky Jon showed in the link has grafted walnut in the buttstock. However, that isn't really a very rare one of a kind thing to see, because a lot of Kalifornia orchard trees are English walnut grafted onto a more disease resistant root stock such as black or Claro. There is a market for grafted blanks for those who want something different, but I'd almost rather look at relatively plain wood or even a synthetic stock.
Most often, when sapwood is visible in a gun stock, it tends to run laterally with the grain rather than perpendicular to it, and it will often be stained to attenpt to match the darker heartwood.
