Michigan Greenstone is a fantastic example of a beautiful regional stone. Its unique patterns and coloration lend it to being collected, cut, and polished wherever rock enthusiasts can get their hands on it. Its rarity also lends it a mystique that simply isn’t there for many stones.
So, let’s learn a bit about what it is and how to find it!
What is Michigan Greenstone?
Michigan Greenstone is called by a few names, including Isle Royale Greenstone, but it’s all the same stuff. The stone is called chlorastrolite, a variant of the common silica-bearing mineral pumpellyite.
The stone itself has a turtle-shell type pattern across it, blending varieties of light green-blue and darker green to great effect. Once polished the stone is incredible, it takes a great polish and develops into something beautiful when handled by a good lapidary. There’s a certain depth to them that isn’t found in many stones, reminiscent of some agate varieties.
The most attractive feature is hard to see in photographs. It also displays low-level chatoyancy, giving the stone’s reptilian patterns a soft shimmer.
It’s also worth $40,000 per pound.
Don’t get too excited if this is your introduction to Michigan Greenstone. The stone invariably occurs as very small pebbles, few could even be called rocks. The largest stone found sits in the Smithsonian, and it’s an astonishing 1 ½” by 3”.
The reason it occurs in such small amounts?
It formed in the air pockets of ancient floods of magma that became basalt. Basalt is a remarkably hard stone, and the air pockets left behind were tiny. When found it’s often as tiny pebbles or even tiny needle crystals hiding in ancient voids.
Complicating manners, its deposition in basalt bedrock and tiny size makes it impossible to dig for on a commercial scale. Basalt is very tight-grained, making extraction of Michigan Greenstone a feat of its own.
Instead, collectors are reliant on the few areas where water has worn through the bedrock and exposed the stone to the light, where it can be found.
In addition to being valuable and rare, Michigan Greenstone can be hard to work. The patterns that play on the surface are usually part of the matrix of the stone, so grinding too deep can ruin the effect. The stone is a conglomeration, so you can also expect variations in hardness along the surface.
It’s either one of the most fun or one of the worst things you’ll ever cut and polish, depending on how much of a challenge you prefer in your lapidary work.
Where Is Michigan Greenstone Found?
On a more serious note, Michigan Greenstone is only found along the shores of Lake Superior. Isle Royale was the original source for this stone, but its conversion into a national park makes collection there illegal.
It’s also one of the few rock laws that are actively enforced. Scofflaws exist across the spectrum in the mineral collecting world and the high value of Michigan Greenstone can bring out the worst in sketchier collectors.
Instead, the amateur would do their best to search along the shores of the Keweenaw Peninsula on the Upper Peninsula of Michigan. It appears that the majority of public land where the stone can be found is pretty much mined out, with old collectors openly stating it’s not what it once was.
Some will collect pieces of basalts with the tell-tale green to take home, rather than looking for nodules on their own. They can be chiseled out carefully by a patient rockhound, and then examined to see if they’re of the right quality to be cut.
They’re hard to find on public land, but the occasional paid dig can be found. They’re usually hosted by lapidary clubs in the area on private land that hasn’t been searched over endlessly and had rock removed to be crushed and used as road base before being hidden beneath asphalt. These can be a bit closed to people who aren’t residents.
Naturally, given the high value and rarity of the stones, there are probably still some good spots on public land that people don’t want to share. That said, my digging has failed to unearth any hidden honeypot and even rockhound discussions about the area seem to focus on how good things used to be.
It’s worth a shot but ensuring you bring home a sample of Michigan Greenstone is going to require some serious homework on location and identifying the stone in the field.
Identifying Michigan Greenstone in the Field
If you happen to find a good spot to dig, you should know there are two ways the stone is found:
- Loose Nodules- Primarily along shores where basalt is being weathered along the surface. You’re looking for tiny pieces of stone in beds of gravel and hoping that you can identify them.
- In Basalt- The other option is to dig into some of the basalt that bears the stone and separate it out. It will appear as small, green specks and possibly nodules visible on the basalt’s surface.
So, your choices are to look for nodules that are often smaller than a dime, search for pieces of broken basalt with Greenstone in them, or to begin hard rock digging where it’s allowed. Adding to the difficulty for the first method is the fact that uncut, unpolished Michigan Greenstone doesn’t look like much at all.
The loose nodules will be incredibly hard to identify. The best will be greenish, but the form doesn’t lend itself to a high natural polish at any considerable sizes. Your best bet is to examine, and re-examine, tons of rough material photographs before heading out.
The nodules still held in basalt are a bit easier to identify. Get the bit of stone wet, brush off any mud, and look for small green-ish specks in the basalt. This is the better way to go.
Be aware that other minerals also occur in the basalt in the area. Of particular note is epidote, which is an olive green stone some may mistake for greenstone at first glance. Most samples of Michigan Greenstone still in the host rock appear speckled and light green.
If you’re used to hunting for jasper and agate, this is a whole new adventure.
Removing Michigan Greenstone From Host Rock
If you manage to liberate a few choice nodules, then you’ll probably want to liberate the Greenstone from the basalt base. It’s really the only way to cut it after all.
You’ll need some tools for this:
- Chisel and hammer
- Diamond Bit Set
Basalt is hard and fine-grained, which makes the whole process a lot more difficult than it appears at first glance.
Use the chisel and hammer on larger portions of basalt, preferably with some distance from the Greenstone. You may even find more nodules inside the basalt as you go, but the idea is to get something manageable and break away any pre-fractured rock.
Liberation from the basalt done closely around the Greenstone is best done with a rotary tool and diamond bits. Keep everything wet, as normal, and use proper PPE for dealing with silica-bearing stones.
You can cut in with diamond bits and burrs to remove basalt. Be careful to touch the Greenstone as little as possible during this process.
Each little speck will present its own challenges for removal, but careful work and a steady hand will eventually yield the stones you’re looking for. Separating them from the basalt is only the start for most of them, but doing so is also a massively rewarding experience!