Larry Lyons: Reader corrects author on Petoskey StonesPublished 9:45am Wednesday, November 11, 2009
An astute reader responded pointing out that Michigan’s official gemstone was the Isle Royal greenstone, not the Petoskey stone. A quick check showed I had, indeed, made an egregious error. The Petoskey stone only holds the lowly title of Michigan’s official stone, not gemstone. You’d think being a fossil it should be our state fossil but that spot was already taken by the mastodon, of which a number have been found here in Michigan. So the Petoskey stone is just a rock, albeit with official status.
Thanks for keeping me on my toes Kay.
Our state gemstone is every bit as unique to Michigan as the Petoskey stone for it is only found in the copper country of the Upper Peninsula’s Keweenaw Peninsula and nearby Isle Royal out in Lake Superior. Actually, the name Isle Royal greenstone is a misnomer conjured up by locals of the area. The term greenstone is generically used for a variety of rocks and minerals.
The true and official name for the mineral is chlorastrolite. It varies in color, most commonly from greenish-gray to bluish-green. Ironically, the pattern in the stone is reminiscent of a turtle’s shell just like the Petoskey stone but for an entirely different reason. The Petoskey stone’s turtle shell pattern comes from the shape of the coral cells. Chlorastrolite’s turtle back appearance is from the finely radiating (stellated) formation of crystals within the stone.
It’s difficult to describe in layman’s language and the technical terms needed to accurately depict this would bore all of us to tears so I won’t even try. Just imagine a blue-green Petoskey stone, or better yet, surf the ‘Net for photos.
Trying to pin down chlorastrolite gave geologists fits for nearly a century. It was first described in 1847 during Michigan’s copper rush but it didn’t receive much attention for the next fifty years.
In 1901 a geologist wrongly associated it with a mineral found in central Europe. That held until 1920 when a Harvard professor disputed that and, again wrongly, put it in a different mineral family. He soon realized his mistake and decided it was a unique new mineral in a class all its own. He named it kearsargeite but that doesn’t exactly roll off the tongue and his sponsors insisted he change it.
He then chose pumpellyite, honoring another geologist active in Michigan’s copper exploration, Raphael Pumpelly. That doesn’t have much of a romantic ring to it either but it stuck, at least sort of for a while. Pumpellyite was used to classify a group of similar minerals. In 1940 a University of Michigan professor declared Michigan’s chlorastrolite to be a unique and specific variety of pumpellyite and that’s where it stands today. Michigan’s state gemstone is officially chlorastrolite.
Chlorastrolite appears to be found only in association with Michigan’s copper veins. The larger specimens are found deep within the bowels of the earth in the copper mines. What’s accessible to the average rock hound are small, wave rounded pebbles along the Lake Superior beaches.
The largest concentrations are on Isle Royal but being a State Park it is illegal to remove them from that location. Therefore pebble perusers focus their efforts on the Keweenaw Peninsula beaches.
Chlorastrolite doesn’t generate much excitement in the gem world. The beach pebbles are very small, about a quarter inch give or take, so are only suitable for stick pins, small earrings and such. It’s often interspersed with impurities such as quartz and its inconsistent structure and hardness often make it difficult if not impossible to polish. So we don’t hold the world in awe with our state gem.
But when you don’t have emeralds, rubies, sapphires, opals and such you make do with what you’ve got. Maybe I was onto something with the thought of the Petoskey stone being our gem after all – nah, don’t think so.
Larry Lyons writes a weekly outdoor column for Leader Publications. He can be reached at firstname.lastname@example.org