An exact age for an old rock

Part of Hall of Planet Earth.

Stillwater Gabro_HERO

This gabbro formed by crystallization of magma. In order to date the rock, the three minerals that make it up were separated from one another. Three vials containing these minerals are shown here.  The white mineral is plagioclase, the green one clinopyroxene, and the black one orthopyroxene. The fourth vial contains the crushed whole rock.

AMNH collection