Thomsonite

specimen number:
1791299

location:
Good Harbor Bay, Grand Marais, Cook County, Minnesota, USA

description:
An alluvial example of thomsonite. Classic for the species and locality.

Good Harbor Bay, located just southwest of Grand Marais along the North Shore of Lake Superior in Cook County, Minnesota, is a renowned locality for the semi-precious zeolite mineral thomsonite. Geologically, the area is defined by Mesoproterozoic basaltic lava flows formed roughly 1.1 billion years ago during the Keweenawan continental rifting event. As gas bubbles and vesicles in the cooling lava filled with hydrothermal fluids, secondary minerals precipitated out over time, creating the distinct nodular amygdules that contain gem-quality thomsonite. Mineralogically, the local thomsonite is famous for its unique concentric rings and vibrant color combinations of pink, red, white, and striking shades of green caused by iron and other trace impurities. Historically, the unique pebbles along the shoreline gained widespread attention following geological surveys in the late 19th century, eventually establishing the stretch of coast as a legendary destination for rockhounds, lapidary artists, and local artisanal gathering.

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