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.
Producer Frank Yablans had been involved in this project since its inception. Michael Crichton had pitched his idea for a modern-day King Solomon's Mines to him, before he had even written the novel. Yablans liked the idea so much that, without Crichton's authorization, he sold the film rights to Twentieth Century Fox in 1979, a year before the book was published. The technology to create the apes was not available at the time, however, and the project never materialized. During the production of Jurassic Park (1993), Crichton was impressed with the dinosaurs that Stan Winston's studio had created. Producer Kathleen Kennedy (who produced both films) suggested using Winston again for the apes, and suggested the project itself to her husband, Frank Marshall, and Crichton agreed. This resulted in Yablans, Marshall and Kennedy collaborating on the film.