The Michigan State Gem is commonly called "Greenstone" or "Isle Royale Greenstone". The correct mineral name is Chlorastrolite, which is a variety of pumpellyite. Chlorastrolite is found chiefly as small rounded pebbles on the beaches of Isle Royale in Lake Superior and also throughout the Keweenaw Peninsula associated with the native copper deposits. The Keweenaw nuggets are found as black to green colored nodules in vesicles of the amygdaloid basalts.
Chlorastrolite was named the "Official State Gem" of Michigan by the 76th Legislature (Act 56, PA 1972). This legislation was introduced by Representative Russell Hellman of Dollar Bay; a longtime enthusiast of the natural wonders found throughout the Keweenaw.
We often hear, "I thought Petoskey Stone was the State Stone?!" Our answer is, "It is!! Petoskey Stone is the State Stone, and Greenstone is the State GEMstone."
Collecting of the "greenstones" from the beaches of Isle Royale has been prohibited since 2000. The best localities now are the waste rock piles which dot the Keweenaw Peninsula in the western Upper Peninsula of Michigan. Primary collecting spots are as follows: Central Mine, Central Exploration, Cliff Mines, Phoenix, Mandan, Delaware, plus many outcrops and one notable area on Eliza Creek near Eagle Harbor. No known outcrops with greenstones occur on the mainland Lake Superior shorelines, but it is possible to find isolated nodules which were deposited by river or glacial actions.
The perception that Isle Royale yielded the best greenstones is somewhat misleading. Nature has done a wonderful job of "highgrading" the selection found on the Island; as all soft, hollow, broken, or inferior rocks have been turned to dust, and we see the better material remaining.
Mainland stones also have good qualities, but a person must process much more material to obtain the same yield as "Island stones". Major differences are the lack of pink prehnite associated with Isle Royale stones, the size of mainland stones is larger, and mainland stones tend to be darker with large chatoyant cell structures.
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Naturally tumbled by Lake Superior
Greenstone nodules embedded in the basalt matrix
Greenstones have a hardness of 5 - 6 and are very well suited for all varieties of jewelry. Polishing is tricky, as some stones have quartz or calcite centers, are hollow, and the most important consideration is the rapidly changing cell and color pattern as a stone is sanded to shape. Soft chlorite inclusions on some of the mainland gemstones are a problem which can ruin a great pattern. The toughness of this stone allows for cutting and polishing of all types of cabachon shapes, calibrated or freeform, and is a great material for inlays.