EduR394 Agate

3D Model

79×49×27mm

Agate is a sedimentary rock that usually forms, quite unintuitively, inside igneous rocks. When the vesicles of an igneous rock, air spaces where gas did not have time to escape the lava before it solidified, fill with a silica-rich liquid long after the igneous rock was deposited, the silica can crystallise out of the liquid as the minerals quartz and morganite.

At the right silica concentration, the minerals precipitate out in tiny crystals not visible to the human eye, in a rock called chalcedony. When these crystals grow in successive layers, usually in an empty cavity like a vesicle, the resulting banded rock is called an agate. When the silica concentration is too high, big quartz crystals will form, like the middle band in this agate, and when the silica concentration is too low, no crystals will form, leaving an empty centre known as a vug, to form a geode. Sometimes, when the liquid stays trapped in the vug, a geode can still have liquid in it when you break it open!