MINERAL OF THE MONTH: September 2012 – Chlorastrolite Greenstone

The mineral of the month is Chlorastrolite, also known as Isle Royale greenstone. This mineral was designated as the official state gem by Michigan’s 76th Legislature (Act 56, PA 1972). This legislation was introduced by Representative Russell Hellman of Dollar Bay. This mineral is a green variety of the mineral pumpellyite, and has a pattern of star-like crystals which forms a turtle-shell pattern. Greenstones are found in the waters and on the shores of Isle Royale, where it is illegal to collect due to the island’s national park status. It can also be found in the Michigan Copper ranges in the western Upper Peninsula. Many of the old tailings left over from the mining era have greenstones in them. They appear as dark green, small round or almond shape nodules in the basaltic rock. In some cases these nodules have weathered out of the volcanic rock and can be found along the shoreline. Unfortunately, only a small percentage of the green nodules are actual greenstones, as most are chloride, prehnite or some other mix of minerals. And of course, of all the greenstones found, perhaps only a few out of every hundred are gem quality.

It is difficult to identify an unpolished pebble of chlorastrolite. Most gem quality greenstones are very small, and it is rare to find one that is larger than a half inch. The largest gem quality stone is in the Smithsonian and measures 1.5 by 3 inches.

Since it is illegal to search for these gemstones on Isle Royal, the best localities now are the waste rock piles located in the old Keweenaw Peninsula mines in the western Upper Peninsula of Michigan. Some possible search sites include the Central, Central Exploration, Cliff, Phoenix, Mandan, and Delaware mines as well as some of the shoreline outcrops near Eagle Harbor.

Leave a Reply

Your email address will not be published. Required fields are marked *