Ironwood is a name applied to many hard, heavy woods. Some ironwoods are so dense that their specific gravity exceeds 1.0 and they are therefore unable to float in water. North American ironwoods include the American hornbeam, the mesquite, the desert ironwood, and leadwood (Krugiodendron ferreum), which has a specific gravity of 1.34–1.42, making it the heaviest in the United States.

The heaviest wood is black ironwood (Olea laurifolia), also called South African ironwood. Found in the West Indies, it has a specific gravity of 1.49 and weighs up to 93 pounds (42.18 kilograms) per foot. The lightest wood is Aeschynomene hispida, found in Cuba, with a specific gravity of 0.044 and a weight of 2.5 pounds (1.13 kilograms) per foot. Balsa wood (Ochroma pyramidale) varies between 2.5 and 24 pounds (1 to 10 kilograms) per foot.