The fish eye in the flintie is a result of the translucent glass in the center being pinched at the poles where the cutter squeezed the ingot and you can see a bit of what's inside. Break a flintie open and you can see the different glass inside. I've seen flinties in white, tan, green, peach, blue, red, purple, yellow, black, and maybe other colors. But the inside is a translucent maybe opal type glass. To resemble flint, which often has a slight translucence as well. Here is a big flintie broken in half to see the inside (dug at Akro).