EGLE staffer offers tips for finding Lake Superior agates, the highly sought-after and elusive stone

Today’s MI Environment article by Mary Ann St. Antoine, a senior environmental quality analyst in the Marquette District Office of the Michigan Department of Environment, Great Lakes, and Energy (EGLE), offers tips on how to find and identify agates, an ornamental stone.

 

Agates fresh out of the rock tumbler.

 

Michigan rock hounding is a favorite pastime for locals and tourists alike.  There are so many pretty, colorful, lake smoothed pebbles found on the beaches of the Great Lakes.  One of the most coveted pebbles is the elusive Lake Superior agate.   

What actually is an agate?

Agates are colorful banded rocks which are mainly composed of chalcedony – a variety of quartz. They’re formed when gas bubbles in lava leave a hole or “vesicle” in the rock. Silica rich solutions flow into the hole in the host rock and fill it up over time, making the bands.  Different chemicals in the silica rich solution cause bands to be different colors. The volcanic rock is weathered away and the harder agates are released from the softer host rock.

Unpolished agate in basalt host rock (l) next to agate showing waxy luster and pock marks (r).

 

The bright bands are a dead giveaway but usually the inside vibrant bands in the agate can’t be seen from the unpolished outside of the rock. Look for a dull waxy luster.  Often, they are red, orange or brown with a pock-marked surface.  Agates are translucent.  Sometimes this is hard to tell from the outside -- hold them up to the light and see if they transmit a little light.

Polished agates showing banding. 

 

Agates are sometimes hard to identify, even for the experts. I found my favorite agate in Grand Marais.  It was rusty brown, pock marked with no bands on the outside.  I almost threw it away as a dud.  I was so excited when I took it out of the tumbler, and it was the most beautiful banded purple amethyst!

My favorite agate!

 

Where can I find agates? 

Find a Lake Superior beach that has lots of exposed pebbles.  A good time to look is after a storm when the waves have washed up new pebbles.  Some of my favorite agate places in the Upper Peninsula of Michigan are Whitefish Point in Paradise, Muskallonge State Park north of Newberry, Grand Marais Beach, Bay Furnace Beach in Munising, Pebble or Dog Beach in Marquette, Agate Beach and Misery Bay in Toivola, Black River Harbor Beach in Ironwood, and Eagle River Beach in the Keweenaw.

Agates can also be found in rivers with pebbles and in gravel quarries.  Surprisingly, many beautiful agates have been found in the landscaping pebbles at shopping centers!

Spend an afternoon walking along a sunny Michigan beach picking pretty pebbles. Maybe you’ll find a keeper.

If you’re not in the mood for looking for agates but would like to see some beautiful specimens, the A.E. Seaman Mineral Museum located on the campus of Michigan Technological University in Houghton has world’s best collection of Michigan minerals including many beautiful agates.   The Ernest Kemp Mineral Resources Museum on the campus of Lake Superior State University in Sault Ste. Marie has many specimens from all over the world as well as many unique local varieties of minerals including agates.

Quiz:  Is it an agate?

Not an agate – has bands but is not translucent. It is banded jasper and hematite. It is also known as Jaspilite and is very collectable.

 

Not an agate – composed of chalcedony and translucent but has no bands.

 

Not an agate -- it has bands but is not translucent. It is laminated sandstone.

 

 

Yes!  An unpolished tube agate.

 

 

Maybe? It has a waxy, translucent, pockmarked luster. There might be some banding. It needs to be cut or polished to see the inside to be sure.

 

Happy hunting!