"Agate" is a rockhound / collector term for cavities in rocks - usually sedimentary rocks such as limestone or igneous rocks such as basalt - that have been partially or completely filled with irregularly concentric layers of microcrystalline, fibrous quartz (chalcedony - SiO2). Agate is quartz. This example is nicely translucent.
The numerous small arcs that appear as shadows are percussion cracks, which formed as clasts collided in water by wave action and storm action.
Locality: loose clast, shoreline of Lake Superior, northern margin of the Upper Peninsula of Michigan, USA
Loading contexts...
