Heartwood
Inner, harder part of the tree trunk — preferred for stair treads.
Heartwood is the inner, older part of the tree trunk. It is harder, more durable and darker than the outer sapwood and contains deposited tannins and resins that make it more resistant to moisture and pests. In oak, the difference between heartwood and sapwood is particularly pronounced. For high-quality stair treads, heartwood is preferred as it is more dimensionally stable and shows a richer grain.