Gator Girl Rocks

Rockhounding Michigan

Michigan is a good state for rockhounding.  The state is famous for its legendary copper deposits in the Upper Peninsula (and Ben Franklin’s negotiating skills) as well as its extensive iron ore deposits.  Michigan also is famous for its Petoskey stones (fossilized coral).  Rockhounders also may find banded agates, jasper, chert, and other specimens.

State Rocks, Gemstones, Minerals, Fossils, & Dinosaurs

Rockhounding Tip:  Knowing state rocks, gemstones, minerals, fossils, and dinosaurs often can be very useful information for rockhounders.  Ordinarily, states with significant mineral deposits, valuable gemstones, fossils, or unusual or significant rock occurrences will designate an official state mineral, rock/stone, gemstone, fossil, or dinosaur to promote interest in the state’s natural resources, history, tourism, etc.  Accordingly, such state symbols often are a valuable clue as to potential worthwhile rockhounding opportunities.

State Rock:  Petoskey Stone [Fossilized Coral] (1966)

Michigan designated the Petoskey stone as its official state rock in 1966 and became the first state in the nation to designate a fossil as an official symbol.  The Petoskey Stone was named the state stone four years before the coral species it is made of was scientifically described.  Petoskey Stones are common in the northern part of the Lower Peninsula, and are actually fragments of Devonian Period coral reefs.  These reefs grew in what is now Michigan about 360 million years ago, when a shallow sea covered the area.  The particular species of coral represented is Hexagonaria percarinata, an example of what is known as a tabulate coral, one of two main types (along with rugose "horn" corals) that made up the reefs of the Paleozoic Era.  Both tabulate and rugose corals were decimated at the end of the Paleozoic, and replaced by the scleractinian corals that are common in reef environments today. 
 Michigan was extensively resurfaced by glaciers during the Pleistocene (the last 1.6 million years), and as part of this process, moving glaciers plucked up pieces of the bedrock and carried them along, smoothing and rounding them in transport.  This accounts for the regular shapes of Petoskey Stones found today.  Good Petoskey Stones may be collected at Fisherman's Island State Park near Charlevoix, and at the south end of the beach at Petoskey State Park.  In the late 1990s, the largest Petoskey Stone yet discovered, an amateur fossil hunter at Sleeping Bear Dunes National Lakeshore found a one-ton monolith.  Most of the Petoskey stones found along beaches and in gravels have already been rounded and smoothed by glacial and water action.  Many of these are suitable for hand polishing and will often take a high polish.  Occasionally a Petoskey stone will be found in "rough" form that has not been subjected to smoothing by glacial or wave action. 

State Gemstone:  Chlorastrolite (1973)

Michigan designated chlorastrolite (also known as greenstone or Isle Royale Greenstone because of its color) as its official state gemstone in 1973.  Chlorastrolite actually is a variety of the mineral known as pumpellyite.  It is found chiefly as small rounded beach pebbles along the shore of Lake Superior and on Isle Royale.  Gem varieties are found only in Michigan. 
 Chlorastrolite is a beautiful green mineral with a chatlyant "turtle-back" pattern.  The name chlorastrolite comes from the words chloros (green), aster (star), and lithos (stone).  It isn’t always green.  Rather, its colors range from yellow-green to almost black. 
Greenstone formed as small vesicle fillings in basalt.  The usual size varies between 1/8 and 1/2 inch (but larger stones are found). What is thought to be the largest chlorastrolite in existence is at the Smithsonian and measures 1-1/2 by three inches. 
You can find gem-quality chlorastrolite at the mine dumps if you look carefully.  Greenstones have been found at most of the mines at Keweenaw; however, some mines produce better greenstones than other mines. The Central Mine, located just off US 41 north of Calumet, has been the source of some of the largest greenstones.  To this day, many persons fail to realize that much of the chlorastrolite occurred under the Keweenaw Peninsula and was discarded on mine dumps by the copper miners when they tunneled to the copper.  The mines are not operating now and new material is not being brought to the surface, but the greenstones are usually overlooked by the rockhounds searching for copper. 

State Fossil:  Mastodon (2002)
Michigan designated the mastodon (Mammut americanum) as its official state fossil in 2002. 
 Mastodons are found as fossils all over the Great Lakes Region and Upper Midwest, with over 250 discoveries in Michigan alone.  They were large members of the elephant family that stood as such as ten feet high at the shoulder.  Many people are confused as to what the difference is between a mammoth and a mastodon.  In short, mammoths were larger and had teeth adapted primarily for grazing, whereas mastodons were browsers (leaf eaters) with multi-cusped teeth.  Mastodon fossils are most often found in regions that were forested during the Late Pliocene and Pleistocene epochs (4 million - 10,000 years ago).  The only known mastodon trackway was uncovered near Saline, Michigan and a cast of it is on display at the University of Michigan in Ann Arbor.


 

Rockhounding Resources

Michigan Geological Survey

 

 

• Dan R. Lynch & Bob Lynch, Michigan Rocks & Minerals: A Field Guide to the Great Lake State (2010).
• Joseph J. Kchodl, The Complete Guide to Michigan Fossils (May 2006).
• Bruce Mueller & William Wilde, The Complete Guide to Petoskey Stones (June 2004).
• Kevin Gauthier & Bruce Mueller, Lake Huron Rock Picker's Guide (2010).
• Bruce Mueller & Kevin Gauthier, Lake Superior Rock Picker's Guide (2007).
• Bruce Mueller & Kevin Gauthier, Lake Michigan Rock Picker's Guide (2006).
• Bob Lynch & Dan Lynch, Lake Superior Rocks & Minerals:A Field Guide to the lake Superior Area (2008). 
• June Culp Zeitner, Midwest Gem, Fossil, & Mineral Trails: Great Lakes States (Rev. ed., June 1999)
• June Culp Zeitner, Midwest Gem, Fossil, & Mineral Trails: Great Lakes States (Rev. ed., June 1999 – originally published 1956).
• Allan W. Eckert, Earth Treasures Vol. 1 - Northeastern Quadrant (1985; reprint in 2000).
• James Martin Monaco & Jeannette Hathway Monaco, Fee Mining & Mineral Adventures in the Eastern U.S. (2d ed. 2010).
• Kathy J. Rygle & Stephen F. Pedersen, Northeast Treasure Hunter's Gem & Mineral Guide (4th ed. 2008).

 

Museums of Interest to Rockhounders

A.E. Seaman Mineral Museum – The Mineral Museum of Michigan
Houghton, Michigan

The museum is the official mineral museum of the state of Michigan.  The museum offers the visitor the opportunity to see the worlds’ finest collections of Keweenaw Copper District minerals, Lake Superior Iron District minerals, Michigan minerals, and one of the best fluorescent mineral exhibits in the United States.  It also offers one of the largest systematic mineral collections on public view in North America.

University of Michigan Museum of Natural History
Ann Arbor, Michigan

Permanent Exhibits include the most extensive prehistoric life collection in the State of Michigan and rock and mineral specimens in the Geology section.

 

Cranbrook Institute of Science
Bloomfield Hills, Michigan

The Institute’s Mineral Study Gallery has over 11,000 specimens and the museum exhibits rocks, minerals, and fossils.

 

Coppertwon USA Mining Museum
Calumet, Michigan

Museum focuses on Michigan’s famed historic copper mining industry.

 

 

Gitche Gumee Agate & History Museum
Grand Marias, Michigan

This small museum exhibits rocks, minerals, gems, and fossils but focuses mainly on Lake Superior Agates.

 

 

Places to Visit - Interesting Sites To See

Isle Royale National Park
Houghton, Michigan

Located on Isle Royale, the largest island in Lake Superior, and nearby islands, the Park includes an ancient Indian copper mining site.  The copper on the island occurs in pure form.

 

Presque Isle Park
Marquette, Michigan
In addition to seeing the 10,000-year-old bog, visitors may see the world’s largest piece of glacial float copper, which is displayed at the park.  The specimen, weighing more than forty tons, is approximately fifteen feet in diameter.  Float copper is formed naturally and is ‘floated’ along by glaciers.  This specimen was discovered in 1997 near Hancock.

 

Gerald E. Eddy Discovery Center
Waterloo Recreation Area – Chelsea, Michigan
The discovery center includes geologic exhibits as well as an outdoor trail with informative specimens.

 

 

Rockhounding Sites for Children & Families

Petoskey Stone
Petoskey, Michigan
Petoskey stones are found almost exclusively on the beaches from Grand Traverse Bay to Little Traverse Bay and gravel deposits up to 30 miles inland on Michigan’s lower peninsula.  They are particularly abundant in the tourist region of the same name, the area around the town of Petoskey.

 

Copper
Delaware Copper Mine – South of Copper Harbor, Michigan
Commercial (fee access) business.  In addition to a tour of a copper mine, visitors may collect copper specimens.

Crystallized Copper
Keweenaw Peninsula, Michigan
Michigan’s Keweenaw Peninsula is famous for its crystalized copper specimens.

Fossils – Devonian
Lafarge Fossil Park, Alpena, Michigan
The Lafarge Fossil Park – located at the Besser Museum for Northeast Michigan in Alpena, Michigan – contains Devonian age fossil material (limestone) donated by Lafarge.  You are allowed to collect fossils.