Lake Superior agates are a favorite among collectors

Several large face-polished Lake Superior agates from Minnesota, and a rough agates from the Mississippi River gravels here in Prairie du Chien. (Steve Van Kooten/Courier Press)
By Phil Burgess
Hidden in the sands and gravels of the Upper Mississippi River Valley watershed and areas surrounding Lake Superior is a colorful semi-precious stone known as the Lake Superior Agate. Determined collectors scour the region’s beaches, sand pits, fields and excavations in the hopes of finding them.
So, just what are they, anyway?
Lake Superior agates are a type of chalcedony composed of microscopic, fibrous twisted quartz crystals usually colored various hues by mineral impurities. They are an extremely tough and durable rock (yes — a type of rock, as opposed to a gemstone). Most of them are banded, though not all. No two are ever alike, which adds to their mystique.
Most Lake Superior agates formed inside gas cavities (vesicles) in 1.1-billion-old lava flows surround Lake Superior. Millions of years of erosion freed them from their lava host rock, and glaciers and melt water floods distributed them far and wide from Minnesota to the Gulf of Mexico. Some ‘Lakers formed in the much older (1.9 billion-year-old) sedimentary Iron Range formations that also surround Lake Superior.
‘Lakers have been the subject of study by both collectors (including this one) and geologists for years now. Their formation and internal features still hold many unanswered questions. Because of their durability and striking colors the smaller, common Lake Superior agates found in sorted decorations gravels are often tumble-polished, and a few are fashioned into jewelry. Larger stones are quite rare and are often simply cleaned, oiled and left in their natural condition. Some are also face-polished on one side. The stones can range in size from a grain of sand up to melon-sized specimens weighing tens of pounds. These larger stones are rare, are highly prized and can fetch prices well up into the four figures among collectors.
Agate collecting is a relaxing and rewarding pastime. Sadly, as with everything else these days most sand and gravel pits are now closed to collectors. But the agates are still out there if you can gain permission to search temporary excavations, fields and other construction sites.