Agate surface. Cut and polished agate surface with colourful patterns formed from mineral impurities that were trapped as the agate formed. Agate is silica (silicon dioxide) that precipitated from liquid inside existing rock. Often, successive layers of precipitation are seen as the deposited silica fills hollows in the rock. The patterns are revealed by erosion or cutting the rock open, and pieces of agate are prized as ornaments. The black impurity here is manganese oxide.