Nor can private counterparties restrict supplies of gold, another commodity whose derivatives are often traded over-the-counter, where central banks stand ready to lease gold in increasing quantities should the price rise.
Testimony Before the Committee on Banking and Financial Services, U.S. House of Representatives July 24, 1998
FRB: Testimony, Greenspan -- The regulation of OTC derivatives -- July 24, 1998[federalreserve.gov]