User:Fossilgal/Petoskey stone/Prismatophyllum

Petoskey stone or "Prismatophyllum" is the State stone of Michigan. In many references it is referred to as Hexagonaria but this is incorrect. Although similar in genus, Hexagonaria is found in western North American, but it can be found as far east as Michigan. The walls of the six-sided Hexagonaria tend to be thicker than Prismatophyllum, the true Petoskey stone. Some other colony corals that are mistakenly identified as Petoskey are "Acrocyathus" and "Favosites" and "Indonesian Colony Coral".

Petoskey stone can be found in many locations in the Upper and Lower Peninsula of Michigan, but most people tend to search for it along the western beaches of Michigan on the Lake Michigan shoreline. Because of over-harvesting, it is getting more and more difficult to find good sized stones on the beaches of Lake Michigan. Those who collect Petoskey have to search elsewhere to find it. This can be challenging, but it is possible to find good qualilty stones inland.

Petoskey coral lived in the Lower and Middle Devonian era. When a complete fossil colony is found, it is a real find. A coral colony "head" can come in a variety of sizes - some as small as 2 inches in diameter to colonies that are 1-2 feet in diameter and still have vestiges of their "root"! A complete colony is composed of many small six-sided cells that are 'indented' in the center and sloping down towards the core of the cell. It still has the appearance it did millions of years ago when it was alive, but now it is fossilized. Neither weather, water nor waves have broken it up and smoothed off it's features. A wonderful addition to any collection.

Petoskey is usually a monotone gray to grayish-brown color, and most often "monotone" in coloration. The best quality stones are those that have had natural crude oil from the environment seep into the stone and color the cores dark. When a piece of such Petoskey is polished it is very beautiful. This is why Petoskey with dark cores is the most prized by collectors and jewelry makers.

Unfortunately, in recent years with the introduction of Indonesian Colony Coral that has been made into beads and cabochons for pendants, there has been more frequency of Indonesian Colony Coral being called "Petoskey". It is important that you learn how to tell the difference between the two if you want to purchase some of these items. Although Indonesian Colony Coral is beautiful when cut and polished, and it makes wonderful jewelry, it is NOT Petoskey! True Petoskey has a round "eye" in the center if each six-sided cell. When you see a "dot" or lack of any distinguishable eye, it is not Petoskey. Also, Indonesian Colony Coral comes in colors such as red, gold or light cream that Petoskey does not come in.

It is important that residents of Michigan know what their Michigan State Stone looks like and also the Michigan State Gemstone, Chlorastrolite, also known as Michigan Greenstone.

REFERENCES _________________________________________________________________________________________________


 * Ida Thompson, 1995, National Audubon Society Field Guide to North American Fossils, Knopf Publishers ISBN 0-394-52412-8