Answer:
Most notably, rubber.
Step-by-step explanation:
Rubber was the most important resource of the Congo Free State, since most of it's revenues derived of its exportation. Ivory was a close second.
By the time it was known as the Belgian Congo, the importance of rubber began to fall. Instead, copper mining became pretty popular. The Union Minière du Haut-Katanga (a Belgian-owned mining company) invested greatly in Congo's copper, cobalt, diamonds, and gold mining.