Vintage Stone Bead Necklace

I've debated on what this stone is and even polled my Instagram followers, but the results have been inconclusive. Any ideas? The beads are smooth and cold; it almost seems like they are marble! The necklace is finished off with a silver magnetic clasp.

All vintage items have been previously worn & loved and may show natural signs of wear, unless otherwise specifically indicated. Measurements taken flat, when applicable. All sales final.