A diamond was a precious gem on Earth.

In 1930, James T. Kirk suggested to Spock that diamonds would be just as easily come by as platinum. (TOS novelization: The City on the Edge of Forever)

Types of currency
materials	acta crystal • gold • latinum • ludugial gold • silver
units	bilbok • cent • credit • darsek • dejebbit • dirak • dollar • drokna • ducat • galorine credit slip • ilec • kern • klon peag • laertian dinar • lek • lita • notch-rock • prime transfer authority • quatloo • sakto • sous • szeket • t'chak • talon • tri-esta