Oldest Rock in the WorldOldest Rock in the WorldAmitsoq gneiss from Ameralik Fjord, Godthaab district, West Greenland is the oldest known rock in the World. It has been dated at 3800 million years, five-sixths the age of the Earth, by the rubidium-strontium method. In this method, the age is calculated from the amounts of strontium 87. produced by the slow, steady radioactive decay of the isotope rubidium 87 in a series of analysed samples.