Dense leafy tree up to 10m, stem and branches often gnarled and twisted, forms dense thickets and lives to a great age. The Post Office tree at Mossel Bay is believed to be 600 years old and is a National Monument. Bark is brown, becoming thick and dark to almost black; young branches covered with fine soft grey to rusty hairs, milky latex present. Thick leathery leaves, shiny dark green above, paler green below, often with rusty brown hairs which rub off with age. Unpleasantly scented flowers, greenish-white, up to 3-4mm long, solitary or in few to many-flowered congested clusters in the leaf axils or sometimes on the older wood; all floral parts in fives.

Fruit is roundish, fleshy up to 1.2cm in diameter, shortly-stalked, solitary or in clusters, smooth purplish-black when mature, containing a milky latex; 1-seeded.

Wood is heavy, hard, strong and durable and has been used as a general purpose timber and for building boats, bridges and mills. An infusion of the bark is said to repel nightmares.

Greek: sideros = iron, xylon = a stick or piece of wood; referring to the iron-hard wood of this species.