Gem Profile- Petoskey Stones and Indonesian Fossil Coral

by Judy Ellis

Petoskey Stones and Indonesian Fossil Coral

This article on agatized fossil coral brings our discussion of agates to a close. I hope I have shown you some varieties of agate that you've never seen before! Fossilized or agatized coral, including Petoskey stones, is a type of agate I discovered in my research for these articles - and I have to say, it is now on my Tucson list for next year!

So what is Fossil Coral Agate?

Let's step back about 350 million years to the Devonian period. The land that is now Michigan was near the equator, not even close to its current location.

A warm, shallow sea covered Michigan's Lower Peninsula, fostering the growth of coral colonies.

Over time and due to a changing environment, the corallite skeletons of the corals were replaced by silica or calcite. This process created agatized fossils or calcified rocks.

About two million years ago, glaciers slid across the old ocean floor, displacing these fossilized corals.

Many of them re-settled in the northern half of Michigan's Lower Peninsula. Today, you can find these coral rocks on beaches and in ditches.

Polished Petoskey Stone, via Wikipedia

The scientific name for this fossilized rock is hexagonaria percarinata. In Michigan, this gray rock is known as a Petoskey stone.

It is named for the city of Petoskey, Michigan, where many of the stones were found. The city was named for the Ottawa Indian chief Petosegay, meaning "Rays of the Rising Sun."

Although the chief was named before the stone, some find significance in the name. They suggest the fossilized coral polyps look like suns and the tentacles are their rays.

In 1965, Michigan declared the Petoskey Stone as its state stone. It became the first state to designate a fossil as its official state stone or rock.

Ella Jane Petoskey, the only living grandchild of Chief Pet-O-Sega, was present as the governor signed the bill.

Petoskey stones are softer than typical agates since they are fossils, rating a delicate 3-3.5 on the Mohs scale.

However, when you find them on the beach, they are very smooth because they have been naturally tumbled by rushing water, sand, and other stones.

Petoskeys can be hand-polished using sandpaper and corduroy. (You can explore page 4 of the linked PDF from the Michigan DEQ for step-by-step instructions.)

Where else can you find fossil agate coral? Florida's state stone, Agatized Coral, is another form.

Florida chose it in 1979; it is a form of silicified coral more similar to petrified wood. This is the only gemstone naturally found in Florida and can be located near Tampa Bay.

Agatized Coral, image via Wikipedia

Indonesian Fossil Coral

Besides Michigan and Florida, Indonesia is possibly the most famous place to discover fossilized coral.

Specific locations for Indonesian coral agate are the mountainous regions of the islands of Sumatra and Java.

According to Joel Ivey, a rockhound, "The development of these coral reefs are related to a single massive rise in sea level back in the Miocene [23-5 million years ago]."

He notes that a sudden drop during a period of volcanic activity is what preserved them.

While Petoskey stones are typically shades of gray, Indonesian fossil coral agate tends to be pale to vivid yellow.

However, it can naturally include a range of other colors, such as white, pink, and orange.

Agate Fossil coral can sometimes be heated to bring greater contrast and brighter colors to the stones.

As you can see on Mr. Ivey's website, many of these fossil agate coral stones are heated in traditional brick kilns.

I have to mention this: while exploring Joel Ivey's website about Indonesian Agates, I found a page featuring polished agates that appear to have letters in them! Can you believe this?

Each of these stones is entirely natural.

It takes a creative and patient mind to spot and slowly polish a cabochon to reveal a letter of the alphabet.

How fantastic!

Resources & Recommended Reading

PetoskeyArea.com
Wikipedia

If you have coffee time and are looking for more to read, I recommend you thoroughly explore these 2 sites by Joel Ively:

Gem Profile- Petoskey Stones and Indonesian Fossil Coral

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