Finding a greenstone:
It is illegal to remove greenstones from Isle Royale National Park, so you will have to look in the Keweenaw Peninsula. Some of the old spoils piles from the copper mine contain greenstones. Watch for nodules dark green, ugly looking nodules within basalt. A good hint is the harder the basalt, the better and gemmier the greenstone may be. When you find a greenstone-included rock you will have to very carefully “pop out” the greenstone, careful not to lose it when you pop it out. I generally take the whole rock to an area where if the suspected greenstone pops out you may still be able to find it. Look also for a glint of chatoyancy in a chipped stone. Don’t expect any large greenstones, but maybe you’ll be lucky. Many of the old dump piles at the mines have already been crushed up and used for road fill in the Keweenaw. When I first started hunting rocks in the Keweenaw 20 years ago there were many large piles of rubble to dig through. 
Now many of those piles are gone, substantially depleted, or privately owned and not accessible anymore. Don’t be discouraged as there are still rock hunting opportunities available. The local rockhounds probably will not tell you their best spots, but will point you to a pile of rock that you can find something. Take your metal detector with you, if you get tired of looking for greenstone you can look for copper. You can rent one at Keweenaw Gem and Gift in Houghton if you do not own one.
Understand that the average Greenstone is half the size of a pea, and any greenstone of a large size is extremely rare. Anything over 5 grams is considered a rarity. The greenstones I feature on my website have been accumulated over a 20 year period by personally hunting them or purchasing old collections.
For every ten possible greenstones I pick up, one may be a real greenstone. The others may be basalt or chlorite balls or some other greenstone “wanabe”. Out of the good stones, one in 100 of those may be a large greenstone and solid enough for jewelry. I guess that is the reason you don’t see Greenstones on one of the home shopping networks that feature the common diamonds, sapphires, rubies, and other “jewelry store” fare. I cannot believe people spend good money on common jewelry store merchandise and are not buying really rare and distinctive gemstones instead. My wife refuses to wear diamonds and such. Of course she understands the economics of artificially inflating values by pretending they are rare (I rant about this enough, she picks up some of what I profess). Dr. Robert Proctor of the History Department of Stanford University commented on how a wonderful Lake Superior agate is much more rare than a common diamond in a keynote oration he gave at the Wonderful World of Agates symposium in 2008. If, as Proctor suggested, many agates are far more rare then diamonds, imagine how rare a one-source gemstone such as Greenstone is.
Working with Greenstone-
So now that I have vented a little and feel better I’ll continue. Cutting greenstones is also challenging. The layer of real chatoyant greenstone is often thin and you must cut the stone so as to not go through this layer and yet remove all the blemishes. Sometimes this is not possible and you have to discard the stone. Other times, I’ll hit a hollow spot; stone ruined. If you find a nice greenstone, never allow someone to cut and polish it for you that is not experienced with working with this material. I know just a handful of cutters in Michigan that really know what they are doing with greenstone. It does not take much to botch this material. Greenstone is not like any other stone when cutting it, and I personally ruined quite a few pieces when I was learning to cut it. Thousands of greenstones later I am confident in my abilities. I custom cut and polish greenstones for people that find them. As an aside, and since you have taken time to read my blog, I will tell you that there are crooks and sham artists that may try to sell you what they claim is an actual greenstone, but in fact may be a piece of basalt, prehnite, or other greenstone look alike. At least one of these sham artists operates a roadside business in the area. So buyers beware. Do some research before you buy, or buy from me. My greenstones are genuine, and I would not sell you any stone I would not be proud to own myself. I offer both fine wire-wrapped slides or pendants and polished individual greenstones that you can work with or have a jeweler craft for you.
I’m proud to say we have the largest selection of Isle Royale Greenstone on the internet, but only a small portion of the greenstones I have available are posted. If you are looking for something in particular contact me and ask. I expect to post up more loose stones in the near future.