The city is one of the 40 largest metropolitan areas in the world and it is one of Africa's only two global cities. While Johannesburg does not form one of South Africa's three capital cities, it does house the Constitutional Court - South Africa's highest court.
Johannesburg is the source of a large-scale gold and diamond trade, due to its location on the mineral-rich Witwatersrand range of hills. Johannesburg is served by O.R. Tambo International Airport, the largest and busiest airport in Africa and a gateway for international air travel to and from the rest of southern Africa.