Rock Info 2

Immerse yourself in the rich history of rocks from Michigan and beyond, with fascinating insights into their unique origin stories, formation, and more.

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Petoskey Stones

Michigan’s state stone was named after an Odawa Native American chief’s son, Petosegay, meaning “Rising Sun” or “Sunbeams of Promise.” They are made of fossilized calcite coral, and are 355 to 415 million years old (100 million years before the dinosaurs). Michigan was covered in warm salt water in ancient times, and 6-sided honeycomb coral colonies thrived. Eventually, over time, they were buried and fossilized. Our glacial bed is over 10,000 feet deep, so we will not run out of Petoskey stones. The best time to find them is in Early spring or after strong winds that churn up the beach, although they can also be found inland. We make all of our products from Petoskey stones we find on the shorelines of Lakes Michigan and Huron.

Fordite or Detroit Agate

Fordite is not actually a stone, but paint built up. When cars were still hand-spray painted in factories, such as Detroit, overspray built up and layers of paint accumualted in areas surrounding the factories. Each color layer of paint was hardened under high heat, thus the swirls and unique patterns.

Pudding Stones

Pudding stones range from the size of a pebble to a boulder, and are around 2.4 billion years old. They are found especially on the shore of Lake Huron and the Ontario peninsula. English settlers named the stone because it reminded them of their favorite fruit pudding back in England. Pudding stones are comprised of red, pink, purple, green, and brown jaspers and black chert embedded in tan colored quartzite. This process took millions of years. During the last ice age, 24,000 years ago, pudding stones were carried hundreds of miles into Michigan and surrounding areas by glaciers.

Petrified Wood

Petrified wood fossilized wood of trees, generally 200-250 million years old. Whole trees of petrified wood can be found in Arizona. The fossil occurs in almost every state and can be found around the world. It formed when fallen trees were washed down rivers and buried under layers of mud and/or volcanic ash. Some were buried too deep to decay. When water seeped down to the trees, silica replaced the air in the wood fibers. Other minerals seeped in to create various colors. It took millions of years for it to form. It is very dense and heavy. In some cases, you can still count the tree rings in the petrified wood.

Thunder Eggs

Thunder eggs are around 60 million years old. They range in sizes of less than an inch up to 40 inches, but they are most commonly baseball-sized. They are found in the Western United States, Brazil, Mexico, Africa, and around the world, but they are Oregon’s State Rock. According to legend, Native Americans named thunder eggs after the Thunder Spirits who dwelt in Mt. Hood and Mt. Jefferson. They were said to hurl them at each other during thunderstorms.

No two thunder eggs are alike. They were formed in gas bubbles within lava which served as mud ball molds. The cooled bubbles were gradually filled by water percolating through the porous rock which carried rich quantities of silica and colorful minerals. The deposits lined the cavity with agate or chalcedony.


A precious stone found around the world, agates are around 400 million years old, and can weigh up to 134,680 pounds. Stone age men used agates for tools. The Egyptians used them to make mortar and pestles. More recently, toy makers in the 1800s used them to make marbles!

Agates have clearly defined branding or patterns and can be multiple colors. They are extremely hard and take a lustrous polish.

There are two main theories on how agates are formed. One theory is that agates were formed by the inflow of mineral rich water solutions into empty vesicle pockets. Another is that they were formed from silica gel clumps in molten lava. Both of these theories are highly simplified! Either way, they took millions of years to form. (Source: “Agates: Inside Out” by Karen A. Brzys, learn more at


Geodes are around 360 million years old. The largest geode ever found was 35 feet in diameter — it was found in Put-in-Bay, Ohio. Geodes occur in the United States, Brazil, Africa, and other countries. They can be found in mud banks, streams, and inland. They are a roughly round shape with bumpy exteriors.

Geodes are formed in two different ways. The first is when gas bubbles are trapped in lava and expand but can’t escape, leaving a cavity when the lava solidifies. They can also be formed in cavities left after fossil shells, minerals, and other nodules are removed from a rock. Geode cavities then fill up with agate, quartz, and other minerals from groundwater. These minerals crystalize inside the cavity to create sparkly white, clear, or colored crystals.


Jasper is around 2 billion years old, and comes in a wide range of sizes. It is found worldwide inland and on lakeshores. Jasper comes in many different colors, with blue being the most rare and red the most common. Sometimes it’s spotted, banded, or multi-colored. They were used by Egyptians for jewelry.

Jasper is formed through deposition of low temperature silica-rich waters, percolating through the cracks and fissures of other rocks, incorporating volcanic ash and a variety of minerals and materials in the process. Over millions of years they are cemented together.

Naming Rocks

Rocks can be named for their color or by the location they were found. For instance, “Azurite” comes from the word “azure,” meaning blue. “K2 Jasper” is azurite in granite found in the K2 mountain range of the Himalayas. Another example is “Biggs Jasper,” first found near Biggs Junction, Oregon.

Some rocks are named after people: “Dolomite” is named after the Frenchman Deodat Gratet de Dolomieu. “Howlite” is named after Canadian Henry How.

In order to name a rock or mineral a miner or individual must submit their data to the International Mineralogical Association which studies, classifies, and then names rocks and minerals.

Rocks are defined as being composed of one or more minerals formed by the action of heat, water, or pressure. There are over 5,000 mineral species. 1,000 minerals were discovered in just the past 10 years. Geology is an extremely complex and fluid science.

A note of interest: Lake Michigan has the most colorful variety of stones on earth, including diamonds and gold!