Nice shot. I don't know why they are called that either. Probably has something to do with the colouring, either the top as we see it here, or maybe the view from the bottom - don't know. If you find out, let us know.
Nice shot. I don't know for sure why they are called that either but I would surmise that it's because the color resemble a Tortoiseshell, the colors are very similar to the colors of the hawksbill tortoise, that is on the endangered species list. (I've seen some guitar picks made from reclaimed shells, they are very pretty.)