The short answer is whatever they could get their hands on, this depending on the eco zone where the Zulu's were living. But, there were two that were much favoured, ie;
1) uMzimbeet ( Milletia grandis) ;
This tree was light grey/green barked with longish dark green leaves and fairly thin spreading branches was generally to be found in the dune forests and ravines in the coastal zone, ie <1000m . This wood was yellow with a black heart and was very prone to blunting axes very quickly. The wood was extremely hard and the canopies in mature trees stood at 10-15m, but spread widely. It bears purple flowers and long dark brown seed pods, which could be heard popping loudly when fully devloped. This tree is grown ornamentally quite extensively in many Natal coastal gardens.
2) Tambotie ( Spirostachys africana):
The dark brown/red textured and oily wood was also hard, but not as much so as (1). The wood could not however be used on cooking fires as the smoke is poisonous, as is the latex. This tree was also found in the lower lying parts of Zululand, generally the bushveld areas, ie <1000m. The tree had rough, fissured grey/brown bark and had quite large trunks. Canopies stood at 6- 9m in mature trees. This wood is used oranamenatlly as well and works quite well on machines.