Olea capensis

Olea capensis (synonyms are "Olea undulata" and O. laurifolia) is also called black ironwood. 
It is an African tree species belonging to the olive family (Oleaceae). It is widespread in Africa south of the Sahara. It grows from the east in Somalia, Ethiopia and Sudan, south to the tip of South Africa, and west to Cameroon, Sierra Leone and the Islands of the Gulf of Guinea, as well as Madagascar and the Comoros. It occurs in the bush, littoral scrub and evergreen forest.
The Guinness Book of World Records lists this tree as the world's most dense wood, with a specific gravity of 1.49. It is known for sinking in water, unlike other woods. It is also the one of the world's hardest woods. The timber resists abrasion and is very strong. It is an excellent turnery wood, widely used for wooden items.