The geology of Wisconsin does not lend itself to a lot of precious gemstones or even nugget gold, but during the 1800s residents unearthed diamonds in areas of moraines – glacial deposits – near southeastern Wisconsin. The largest of these diamonds, the Theresa diamond, discovered in 1888 close to the Green Lake Moraine near Kohlsville in Washington County, weighed 21.5 carats and ended up cut into 10 separate stones by its owner. A gem is essentially any mineral, rock or petrified mineral a jeweler can cut or facet and polish for a jewelry setting. Traditionally, precious gemstones include diamonds, sapphires, rubies and emeralds, while all other gemstones fall under the category of semi-precious.
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Diamonds found in Wisconsin occur in glacial deposits and alongside streams and rivers in gravel beds. The glaciers ground up most of the gold into fine, flour gold deposits scattered in stream and riverbeds.
Madison diamonds, named for these precious gems found in Dade, Waukesha and Washington counties include the infamous Eagle diamond and the Theresa diamond. The Eagle diamond from Waukesha County, heisted from New York's American Museum of Natural History in 1964 and discovered by a farmer, weighed 16.25 carats when found. Kenosha County is home to many smaller diamonds, with the discovery of a diamond-bearing lamprophyre diatreme – a volcanic-type tube housing diamond-bearing igneous rock or kimberlite – covering close to 50 acres.
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Placer gold is rare in Wisconsin because most of the gold in the area was crushed into fine flour-type gold during the glacial movement that helped to form the Great Lakes. The best places to find flour gold include areas of already discovered gold like Ashland, Bayfield, Clarke, Dane, Douglas, Dunn and other counties as listed by the University of Wisconsin (see References). Placer gold, found in streams and having rounded edges, comes from placer deposits or gold veins exposed because of erosion. This washes the placer nuggets into streams and rivers for gold-panners to find. You might find small bits of placer gold in Polk County, lodged in sediment and rocks, when rivers and streams run low.
Minerals and Semi-Precious Gems of Wisconsin
You can find Wisconsin quartz in multiple counties in Wisconsin, along with other minerals including andalusite, azurite, beryl, calcite, celestine and many more. You can also find Lake Superior Agate in select areas of Wisconsin, close to deposits near the River Falls in St. Croix County.