Hi Floor! It's not shell, though I know exactly what you mean about the color. I recently bought myself a fun new toy, an electronic gem tester! So now of course I'm running all over the house testing everything that I can find that might be gemstone (and have found some nice surprises!) This bead registers in the area of chrysoprase or quartz. Probably an agate of some type, but the color is very different from any agate I've handled so I'm stumped and thought maybe someone would recognize it. It came in a bunch of ancient-20th century beads that I've been rummaging through. Thanks for taking the time to reply! -Melissa