Along the shores of Lake Superior, agates are a common find. These red-banded chunks of chalcedony have captured the imagination of people since time immemorial and their name holds even when they’re found far to the South as glacial debris. While common, they remain valuable and prized by rock collectors.
So let’s talk about the biggest one that’s been found, and what makes these agates so unique.
How Big Was the Largest Lake Superior Agate?
There is no “official” record in this case, but generally, the largest up for consideration is a 108-pound agate. It currently resides in the front room of the First National Bank of Moose Lake in the town of Moose Lake, Minnesota.
It was found in the gravel pits in the surrounding area. The man who found it, named Paul Ward, was an agate collector but on that day he was just grabbing gravel to fill in some new construction. Digging deeper in, he found the enormous agate.
The agate was a source of contention with his wife as it drew a bit too much attention. Worried about a break-in to steal it, Ward decided to sell it. It was later sold to the group that would become a local Gem and Rock Club in Moose Lake for the princely sum of $400.
From what I’ve been able to ascertain, most of the gravel fields in Moose Lake are pretty well picked over these days. They were originally the result of people clearing their land of rocks, but collectors soon recognized them as a great place to look for agates.
It’s not stopping anyone, but the days of regularly finding fist-sized agates appear to be at an end.
The stone itself is still in its rough form, unpolished, and held on a pedestal in the front of the bank. Part of the agreement for sale is that Ward wanted to be informed if it was resold so he’d know where it was.
What Makes Lake Superior Agates Distinct?
Lake Superior agates generally consist of high-quality chalcedony with red and white banding. The red ranges from a deeper brown like the sard varieties of carnelian to a bright crimson depending on the impurities in the chalcedony.
Chalcedony itself is a form of quartz. Or, more specifically, it’s cryptocrystalline silica comprised of a mixture of quartz and its polymorph moganite. These two crystalline forms of silica grow into each other and interlock on a scale that’s almost impossibly small. Generally distinct crystal viewing requires a thin slice and a scanning electron microscope.
As a general rule, red will be the dominant color in these agates. They’re formed in the bed of basalt beneath Lake Superior and over time iron has leached into the stone. This is generally what creates the red layers in these agates.
In other cases, they may have orange or yellow coloration, which also seems to stem from ferrous impurities in the chalcedony mixture. For instance, the yellow form is generally considered to be colored by the iron ore limonite. On rare occasions, other elements such as copper have rendered parts of these agates blue or green.
Most of the various subtypes of agate are based on patterns and inclusions. Most also have some form of representation in agates from the Lake Superior area.
- Fortification Agates- The most common type found is simple fortification agates. These consist of differing bands of color around a central node. When they have a macrocrystalline center of quartz, they’re often called “geode agates” instead.
- Eye Agate- One of the most sought-after variations, eye agates form as spherical nodes of alternating colors found in a chalcedony matrix.
- Waterlevel Agate- These agates consist of several straight lines of differing colors, generally in the blue and white range. They’re sometimes found as a “mosaic” in Lake Superior agates which consist of horizontal but broken lines of differing colors.
- Plume and Moss Agates- These agates have internal structures of some kind of other minerals, often things like hornblende. The plumes and mosses from this region are generally yellow to red.
You can sit down and dissect agate structures all day since they’re all the same mineral for the most part. The above are just a few of the many recognized types, but they cover the majority of more unique formations found in these beautiful stones.
Lake Superior agates are thought to be one of the oldest agate formations in the world. They formed in the basalt bedrock left behind by ancient volcanoes, forming in hollows in the basalt long after it had flowed and cooled.
It’s important to remember there’s an enormous amount of basalt under the Great Lakes, which also means there’s a large number of these agates. They’re commonly found whenever someone grabs a shovel in the area.
That said, they range from fairly dull to incredibly impressive and the prices are likely to match the patterns and rarity of the individual specimen. While commonly found as random “tumbles” high-end Lake Superior agates can fetch hundreds to thousands of dollars per pound.
Where To Find Lake Superior Agates
Most of these agates can be found along the shore of Lake Superior and within connecting waterways. Here they’ve often been rough tumbled by time, sand, and water which makes them easy to recognize when wet.
In other cases, however, they can be found anywhere a glacier passed along in the past. They’re the state gemstone of Minnesota, but the gems themselves can be found in Minnesota, Michigan, Nebraska, Iowa, and Kansas. These stones were ripped from the basalt bedrock by the passage of ancient glaciers before being released as the ice structures receded.
While your best bet remains closer to the lake, sizable specimens have been found all over a large portion of the United States.