Isle Royale greenstone, scientifically known as chlorastrolite, is the official state gem of Michigan. The mineral is a variety of pumpellyite, formed within the ancient lava flows of the Lake Superior region. Its desirability among collectors and jewelers stems from its distinct optical properties and limited geographic distribution. This guide details the mineral’s characteristics, where it is primarily found, and the legal rules governing its collection.
Identifying Isle Royale Greenstone
The defining characteristic of Isle Royale greenstone is its intricate surface pattern and color. It typically presents in shades of deep green, ranging from dark olive to greenish-black. This color comes from its mineral composition, a hydrous calcium aluminum silicate hydroxide that includes magnesium and iron.
The gem’s unique feature is a mosaic or segmented appearance, often described as a “turtleback” pattern. This visual effect is caused by the finely radiating, fibrous crystals of chlorastrolite. These fibers also exhibit chatoyancy, a shimmering “cat’s-eye” effect where a band of light moves across the stone as the viewing angle changes. The stone is moderately hard, measuring between 5 and 6 on the Mohs scale, making it suitable for jewelry. Most gem-quality pieces found are quite small, often occurring as rounded pebbles less than a half-inch in diameter.
Geographic Distribution and Key Collection Areas
The occurrence of Isle Royale greenstone is confined almost entirely to the Lake Superior region, particularly Michigan’s Upper Peninsula. The mineral formed within the gas cavities, or vesicles, of basaltic lava flows associated with the Midcontinent Rift System, dating back over a billion years. Over time, wave action and erosion have liberated the greenstone from the surrounding basalt.
The namesake location, Isle Royale, served as the type locality where the mineral was first described in 1847. However, the vast majority of collecting now occurs on the mainland, primarily along the Keweenaw Peninsula in Houghton and Keweenaw Counties. Specimens are often found as nodules in the amygdaloidal basalts or in the waste rock piles, known as tailings, of historic copper mines. Key mainland collecting spots include the areas around former mining operations such as Central and Delaware. Lake Superior beaches in the Keweenaw also yield specimens that have been naturally polished by water and sand.
Legal Parameters for Collection
Collecting Isle Royale Greenstone is restricted, especially at its most famous location. Removal of any natural object, including rocks and minerals, is strictly prohibited within Isle Royale National Park. This prohibition is a federal law aimed at preserving the park’s resources and has been actively enforced since the park’s establishment in 1940.
Outside of the National Park, collecting rules vary depending on land ownership. On state-owned lands, including state forests and public beaches, the Michigan Department of Natural Resources (DNR) imposes a strict annual limit. An individual is allowed to remove no more than 25 pounds of rocks, minerals, and fossils per year for non-commercial hobby use. This limit applies to all state-owned land.
Collecting on Great Lakes shorelines is permitted up to the Ordinary High Water Mark. However, the land immediately above this line is often private property, and collecting on private property requires explicit permission from the landowner. Regarding tools, collecting is limited to surface gathering or the use of hand tools; large-scale digging is prohibited unless authorized. Collectors must respect boundaries, posted regulations, and ensure any disturbed material is replaced to minimize impact.
Commercial Availability
For those who choose not to collect specimens, commercial acquisition provides a reliable alternative. High-quality, gem-grade greenstones used in jewelry originate primarily from the reclaimed tailings of historic copper mines on the Keweenaw Peninsula.
The most common sources for purchasing the stone are specialized rock and mineral dealers, particularly those located in the Upper Peninsula of Michigan. These dealers and local jewelry stores offer both rough specimens and finished pieces. Purchasing is the most practical way to acquire larger or expertly cut and polished stones, which effectively showcase the chatoyancy and “turtleback” pattern.