Oak, with a Brinell hardness of approximately 34 N/mm², is one of the hardest native timbers and by far the most popular wood species for stair treads. It features a distinctive grain, high resistance to scratches and dents, and natural durability (class 2). Oak darkens slightly over time, gaining character. Through various surface treatments — oiling, smoking, liming or brushing — a wide range of appearances can be achieved: from light Scandinavian to dark rustic. The heartwood of oak, the harder inner part of the trunk, is preferred.