It’s my second day at the Tucson Gem Show and I’m standing in front of the Expo Center, examining the fossilized shell of the world’s largest giant killer clam. It’s gray-white and smooth, with a lip ...

They form from the inside out. A thin tissue that adheres to the inner surfaces of the shell, called the mantle, and a thickened rim of muscular tissue at the mantle edge deposit new shell material at ...