Lake Superior agate. Agate is an aggregate of different forms of silica (SiO2) that have formed concentric bands within a geode. A geode forms when th

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Lake Superior agate. Agate is an aggregate of different forms of silica (SiO2) that have formed concentric bands within a geode. A geode forms when the silica precipitates from liquid onto the inside surface of an empty pocket in a host rock. The outer layers are variegated forms of chalcedony (amorphous or non-crystalline silica), with the layers becoming more crystalline towards the centre. These agates are stained by iron and found on the shores of Lake Superior, USA.

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