Brazilian Agate Geodes

Individual shapes and minerals vary but your item will resemble the pictured example.

Agate is a microcrystalline variety of quartz characterized by its fine grain and bright color.  It if usually associated with volcanic rock, but can form in many other rocks, like metamorphic rock.  Agate forms in rock cavities, which fill with layers of silicate overtime creating its striking banded pattern, and Brazilian Agate is usually found as sizable geodes like these.  It has historically been the stone most used for handstone carvings as well as mortars and pestles to crush other materials because of its hardness.