What did Mafatu discover that made him cry,"we're rich!" in "Call It Courage"?
Mafatu discovers the remains of the skeleton of a whale.
Mafatu is wandering far down the beach on the island he and Uri have settled on. His heart suddenly gives "a leap of joy, for there, white-gleaming in the sun, was all that remained of the skeleton of a whale".
When he finds the whale's carcass, Mafatu's mind is overwhelmed with the possible items of survival he will be able to fashion from it. Among other things, he envisions "fishhooks galore, splintered bone for darts and spears, a shoulder blade for an ax".
The skeleton is "a veritable treasure", and in his excitement, Mafatu jumps up and down and shouts to Uri, "we're rich," because his find is more valuable than gold. Mafatu and Uri are indeed rich; they now have the raw material that will enable them to make much-needed implements, and their chances of survival are greatly increased.
Mafatu wastes no time in collecting his newfound treasure. He ties as many bones as he can into two bundles, one for him and one for Uri, and the two drag their find happily back to their camp. Mafatu wastes no time in beginning to fashion the item he needs most - a knife. Working until his hands are raw and blistered, he creates "a splendid knife", ten inches long, its handle a knob of joint. The knife would be strong enough to cut the fronds of coconut trees, and would be "a fine weapon as well" ("Drums").