The Gem of Lake Superior

(Apologies for the hiatus.  I was moving and both time and internet were iffy)


The Lake Superior agate became Minnesota's State Gemstone in 1969.  It defeated some tough competition, including Binghamite and silkstone, jaspers rich in iron that reflect Minnesota's iron ranges; Thomsonite, found only in Minnesota along Lake Superior; and Pipestone, which was made into peace pipes by Indians in the area of the same name.  The Lake Superior agate was chosen because it was widespread throughout Minnesota, captured the importance of iron, and tells the story of Minnesota's geological history.  And I would add it's just plain pretty.

Prehistoric Origins: From Fire and Ice

Over a billion years ago, when all life was still microscopic and the atmosphere was just beginning to accumulate oxygen (thanks to a new innovation called photosynthesis), the area that would become Minnesota was splitting apart.  In the midst of the catastrophe, lava rich in iron flowed up from the fissure that would eventually become the trough of Lake Superior.

The massive lava flows cooled, fraught with small bubbles of water and carbon dioxide (these bubbles are called vesicles).  Groundwater eventually began to flow through the rock, carrying dissolved minerals (including ferric iron and quartz) that crystallized onto the inner walls of these bubbles in layers of a silica mineral called chalcedony.  Lake Superior agates are unique because their bands are red or orange, a color they picked up from the oxidized iron.

Over time, some of these agates were released from their bubble prisons by erosion of the softer lava around them.  Many more were scraped up by the glacial ice that arrived 15,000-10,000 years ago with the Great Ice Age. These were dragged south, tumbled and polished as they went along.

So You Want to Be a Collector

Agates are widespread in Minnesota, giving every Minnesotan (or interested visitor) a chance to collect.  On the surface they can be found in northeastern and central Minnesota into extreme northwestern Wisconsin, and underground they are expected to stretch even farther.  Because of their glacial rearrangement, as well as subsequent movement down streams and rivers, agates can be found in much of Minnesota.

While the shores of Lake Superior are often the most popular location to search for agates, these picked-over areas may or may not yield many finds.  Avid collectors have the best luck after a big storm, which tends to turn up new pebbles, and on a calm, sunny day when agates may be easily glimpsed through the still water.  Agates can also be found in a wide variety of rocky areas: lakes and streams, gravel pits, and rural gravel roads (usually made from local rock).  True hunters avoid granite or limestone and search more diligently in areas of basalt and quartz.  Even so, many agates are found by accident in fields and backyards.

What to Look For

Agates are sometimes hard to find, as their bands may not be visible.  They are generally round (molded in round bubbles in the lava), with pitted surfaces from other softer rock in the vesicles and a reddish or yellowish color from the iron.  Sometimes layers may have broken off, leaving a peeled appearance.

The most common type of agate is the fortification agate, which features concentric bands.  Rarer types include eye agate, which boasts round bands on the rock's surface, waterwashed agate, which has an almost perfectly smooth natural surface, and moss agate, which contains tree-branch shapes.  Most agates are pea-sized, but a few up to 20 Lbs (about the size of a bowling ball) have been found.  An agate that is greater than 2 Lbs with perfect shape, color, and banding is called an "all-timer."


You can learn more about agates and Minnesota's geological history at the Moose Lake Agate & Geological Center.

References/Credits