Mahogany planks are the highest level planks used in the Construction skill. Making items with mahogany planks gives 140 Construction experience per plank used. Mahogany planks can be made from Mahogany logs at a Sawmill for 1,500 coins each. They can also be found as a reward from hard Treasure Trails.
Mahogany planks may be used to make furniture in a player-owned house. Mahogany planks are the most expensive of the four types of planks, and are used to make the highest-level wooden furniture items.
The best item to make with mahogany planks to train Construction is a Mahogany table, which grants 840 experience. Building and removing mahogany tables over and over can lead to upwards of 920k experience per hour, with 953k/hr being the theoretical perfect rate.