Know Your Rocks

Isle Royal Greenstone (Michigan Greenstone)

A variety of Pumpellyte, Chlorastrolite (scientific name) is the distinctive Official State Gemstone of Michigan. Greenstone (common name) is also referred to as Isle Royale Greenstone.

Greenstone is also one of the most rare Gemstones on Earth, found only in a small area in the Copper Country of Michigan’s Keweenaw Peninsula and on Isle Royale. Because Isle Royale is a National Park, removing Greenstones from that source is illegal. Some old collections of Greenstone from Isle Royale, before it was a National Park, do appear on occasion, but most of the Greenstones found today are found within the old Copper mine spoil piles in the Keweenaw.

Greenstones are formed in the vesicles (bubbles) in Amygdaloidal basalt.

Greenstones are becoming even more scarce, since many of the old mine waste piles are being crushed and used for road fill. Other mine waste piles are now in private hands and are off limits for rock hunting.

Searching for Isle Royale Greenstone is like finding a needle in a haystack. By looking through tons of rock, perhaps a piece of basalt containing greenstone nodules can be found. In this case the Greenstone can be extracted by careful chiseling or the host rock can be taken home and the Greenstone can be extracted by careful cutting. Sometimes a Greenstone can be found liberated from the host rock through years of erosion. Only about one in on hundred Greenstones found is a high quality Greenstone, and only one in 500 is a larger stone of good quality.

The quality of a Greenstone is based on size, pattern, color, and the Chatoyancy of the gemstone.