1) The greenstone is Michigan's state gemstone. Most people recognize the Petoskey Stone as Michigan's mineral representative, as it is Michigan's official state stone. The greenstone got to share the glory in 1972 when it was officially declared the state GEM.
2) The Michigan Greenstone is also known as the "Isle Royal Greenstone". They are most commonly found on as small rounded pebbles along the shores of Lake Superior, though collection of these little gems on Isle Royal is now illegal. The mineral name of the greenstone is Chlorastrolite.
3) Even though greenstone collection is prohibited on Isle Royal, the gems can still be found in the copper waste rock piles of the Keweenaw Peninsula of the UP. These mainland stones are found embedded in Basalt and are often larger and darker than the Island stones. Often possessing large chatoyant cell structures, the patterns are bolder as well. The identification and extraction of these mainland stones are much more difficult, though it is what rock collectors must go through now that the Isle Royal stones are protected.
4) Isle Royal greenstones aren't necessarily better than mainland stones, they simply are naturally tumbled by Lake Superior thus are much easier to find as they are already somewhat polished. The inferior stones have already been eroded away, leaving the best selection of gems to choose from. Mainland stones however can offer much larger pieces, as well as more complex patterns.
5) Often the "island stones" feature pink prehnite reminiscent of the color of Lake Superior Agates. Some stones have some stones have quartz or calcite centers. Others are hollow and the color and pattern tends to change throughout the depth of the stone. All of these factors should be considered when cutting and sanding the gems.
Information sourced from Keweenaw Gem & Gift