How do you know a bead is ancient

Idar oberstein old carnelian trade beads & agate dzi ancient handmade necklace idar-oberstein is a small town of roughly 50,000 inhabitants in a narrow, picturesque valley in the hunsruck mountains on the very western edge of germany. The huge focal Vintage Tibetan Agate Dzi Bicone Bead in the center measures 50mm x 18mm.