Rai, or stone money, are large, circular stone disks carved out of limestone formed from aragonite and calcite crystals, Rai stones were mined in Palau and transported for use to the island of Yap, Micronesia. They have been used in trade by the locals and are described by some observers as a form of currency.
Rai stones are circular disks with a hole in the middle. The size of the stones varies widely: the largest are 3.6 meters (12 ft) in diameter, 0.5 meters (1.5 ft) thick and weigh 4 metric tons (8,800 lb). The largest rai stone is located in Rumung island, near Riy village. Smaller rai stones might have a diameter of 7 – 8 centimetres.
The extrinsic (perceived) value of a specific stone is based not only on its size and craftsmanship but also on the history of the stone. If many people — or no one at all — died when the specific stone was transported, or a famous sailor brought it in, the value of the rai stone increases.
Rai stones were and still are used in rare, important social transactions such as marriage, inheritance, political deals, sign of an alliance, ransom of the battle dead or, rarely, in exchange for food. Many of them are placed in front of meetinghouses or along pathways.