Director Frank Marshall originally intended to use the computer work pioneered on Jurassic Park (1993) for the gorillas, but opted for models as the computers weren't capable of reproducing hair.
The "diamonds" used for the scenes during the climax of the movie were actually Herkimer Diamonds borrowed from the Herkimer Diamond Mines of Middleville, New York. They are doubly-terminated (two-ended) quartz crystals that are found in only two places in the world. They were the only gems that would look enough like diamonds and be that large.
The diamond that was thrown out of the balloon at the end of the movie was lost, and so could not be returned to Herkimer Diamond Mines.
Dylan Walsh plays Dr. Peter Elliot, a primatologist who's returning a gorilla to the wild. The real Peter Elliot is a gorilla choreographer and ape performer in many movies, including this one.