It is stone money. Although the production has been ceased in 1931, it is still in use for purchase of land. The stones can weigh up to 5 metric tons and are not moved. People just memorize who the current owner is.
The stones are made of aragonite and calcite, two minerals which do not naturally occur on Yap. So stones have to be brought from Palau, an island about 400 km away, which requires a boat trip of 5 to 7 days.
The value of each stone is determined by a series of factors: size, form, natural beauty of the material, age and history of the stone, difficulties in manufacturing, social status of the people involved or simply whether someone has been injured or died during production or transport.