You cannot legally take one. Isle Royale is a National Park, and collecting specimens there is illegal, so this is a stone to look at and not to pocket.
Chlorastrolite, the Isle Royale greenstone, is a green to bluish-green stone with finely radiating or stellate masses that form what collectors call a turtleback pattern. Those masses are chatoyant, so the lustre shifts as you turn them, anywhere from subtranslucent to opaque. It is a variety of pumpellyite, and it was once thought to be an impure prehnite or thomsonite.
It forms as amygdules and fracture fillings inside basalt. Where water and wave action have worn the basalt away, the stones turn up loose as beach pebbles and granules. It is found on the Keweenaw Peninsula of Michigan's Upper Peninsula and on Isle Royale in Lake Superior. An unpolished pebble is genuinely hard to identify, most gem-quality pieces are tiny, and anything over half an inch is rare. The largest gem-quality stone known is in the Smithsonian and measures 1.5 by 3 inches.
The naming is a good lesson in how mineralogy actually works. C. T. Jackson and J. D. Whitney first described chlorastrolite from Isle Royale in 1847. Much later it turned out to be the same material as pumpellyite, described from the Keweenaw Peninsula in 1925 and named for Raphael Pumpelly. Because the peninsula material had been described in far greater detail, pumpellyite was already entrenched in the world literature before anyone realised the two were identical, so pumpellyite is now the only valid species name. Chlorastrolite survives as the term for the variety with the Isle Royale habit, and as the USGS puts it, pumpellyite is common in many parts of the world while the chlorastrolite variety is apparently rare outside this island.
One warning about a neighbouring stone. Most of the so-called thomsonite from Isle Royale is actually pink prehnite, including the material from Thomsonite Beach on the north side, which is why Isle Royale thomsonite has always been rated inferior.
Chlorastrolite is the official state gem of Michigan.
Chlorastrolite
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