So what is Fossil Coral Agate?

Let's take a few steps back in time. In the Devonian period about 350 million years ago, Michigan wasn't even in the same zip code (so to speak): the land that is now Michigan was near the equator, and a warm, shallow sea covered Michigan's Lower Peninsula and fostered the growth of coral colonies. Time and changing environment caused the corallite skeletons of the corals to be replaced by silica or calcite, becoming agatized fossils or calcified rocks. As glaciers slid across the old ocean floor, about 2 million years ago, the fossilized corals were displaced and many re-settled in the northern half of Michigan's Lower Peninsula, where you can find the coral rocks on beaches and 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, named for the city of Petoskey, Michigan, where many of the stones were found. Petoskey was named for the Ottawa Indian chief Petosegay or Pet-O-Sega, which means "Rays of the Rising Sun." While Chief Pet-O-Sega was named before the stone was associated with him, some see its significance, as if the fossilized coral polyps are suns and the fossilized tentacles are their rays. In 1965, Michigan declared its state stone the Petoskey Stone, becoming the first state to set a fossil as its state stone or rock. Ella Jane Petoskey, the only living grandchild of Chief Pet-O-Sega, was present as the governor signed the State Stone bill.

Petoskey stones are softer than typical agates because they are fossils, a delicate 3-3.5 on Mohs Scale, but when you find them on the beach, you will discover that they are very smooth. This is because they have been naturally tumbled by rushing water, sand, and other stones. Petoskeys can be hand-polished with sandpaper and corduroy (explore p. 4 of this PDF download for a step-by-step instruction, by the Michigan Department of Environmental Quality, Geological Survey Division)

Where else can you find fossil agate coral? In the US, Florida's state stone, Agatized Coral, which it chose in 1979, 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 of Indonesian coral agate are the mountainous regions of the islands of Sumatra and Java. According to Joel Ivey, a rockhound who has spent decades traversing Indonesia, Thailand, Alaska, and other locations "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] and a sudden drop during a era of volcanic activity preserved them."


Would you have guessed what lay inside this spongelike rock? Here is a beautiful peek into a Fossil Coral Agate from South Sumatra, Indonesia, courtesy of
Joel Ivey