The Crystal Birds of Krypton
The Crystal Birds were magnificent huge birds with skeletons and flesh composed of rock crystal. Similar in size to the "Rocs" of Earth legend, these birds became extinct on Krypton—but when they existed, they flew to a specific area of the planet when close to death and their carcasses composed part of the Jewel Mountains. The story of their demise is recounted to Superman by Jax-Ur after his quest to find Jewel Kryptonite in Krypton's past (Act No. 310, Mar 1964: "Secret of Kryptonite Six!").