Densewood

One of the most important products of the forests of Aerenal, densewood is a strong, hard, heavy wood similar to iron in its properties. It has hardness 8 (comparable to stone) and 20 hit points per inch of thickness. The DC for breaking a densewood item with a Strength check increases by 5 compared to a normal wooden item.
A densewood item weighs twice as much as the same item made from normal wood, and costs twice as much (before adding any cost for a masterwork component or an enhancement bonus). Items without wooden parts, including armor and bladed weapons, cannot be made from densewood.

Price: x2 normal cost

Source: Eberron Campaign Setting