The fat, gray "beads" in the jewelry are actually seeds. Commonly called "sea beans," they are the Gray Nickernut from a tropical, prickly shrub that grows wild on Nevis (Caesalpinia bonduc).
The bush has menacing, curved thorns that dig into your skin and stay - like a fishhook - and the pod itself is covered in thin, sharp thorns that pierce the skin at the slightest touch. Obviously they are a real pleasure to harvest - but so beautiful and unique!
Locally they are named "Firestones" because when rubbed on a hard surface they heat up and it is a fun game for children (and fun - loving adults!) to burn each other! Also known as "Donkey Eyes" on some islands, they are the traditional playing pieces for an old African game called Oware, which traveled with the slaves to the Caribbean and is still played today. Adults reminisce pitching them like marbles and call them "Knuckle Stones."
These seeds are natural, with only a hole drilled in them for stringing. Keep them relatively dry.