Thomsonite

Thomsonite was created from lava flows of the Keweenawan Period, over 600 million years ago. Gases trapped within the lava and seams between the basalt lava flows turned into hollow pockets when the lava hardened. Over hundreds of thousands of years, these hollow openings filled and solidified, forming Thomsonite.

The unique combination of volcanic activity and certain chemicals and minerals is responsible for why the gemstone formed in this location along the North Shore.

Thomsonite is a mineral belonging to the zeolite group of minerals, which has over 35 different recognized members. Thomsonite is one of the rarer zeolites. It forms tight acicular radiating clusters and sphericules as well as some blockier crystals and is found in the vesicles or bubbles of volcanic rock, as are most other zeolites.

Where to Find Thomsonite

Thomsonite can be found locally on the beaches between Lutsen and Grand Marais, on Isle Royal, Michigan and near Saxon Falls and the Montreal River Gorge on the Keweenaw Peninsula in Michigan. The color and texture of Minnesota’s Thomsonite makes it unique as gem-quality stones are found only in a limited area of Lake Superior’s basalt shoreline about 5 ½ miles southwest of Grand Marais. Thomsonite Inn is located in the middle.

Thomsonite is also found in the Kilpatrick Hills of Scotland; Northern Ireland; Saxony, Germany; Russia; the Faroe Islands; Kern County, California; and Cape Lookout, Oregon.

Colors

Pure Thomsonite is snow-white and sometimes translucent. Other compounds such as ferric and / or ferrous iron or copper are responsible for the various colorations within the gemstone. Commonly found colors are pink, tan, white, red and brown. Those with green, gray or black backgrounds or green eyes are the most highly prized and least frequently found.

History & Background

Thomsonite was first described in 1820 by Scottish mineralogist Thomas Thomson, who found the mineral in volcanic rocks on the Isle of Skye in Scotland. It is a hydrated sodium, calcium, and aluminum silicate mineral. It forms in radial or botryoidal patterns with a silky or pearly luster.

Thomsonite crystals may be transparent to translucent and can be found in a range of colors, including white, pink, green, yellow, and orange. Its crystal structure is characterized by interwoven rings of silicate tetrahedra, giving it a unique appearance and optical properties.

The Minnesota Geological Survey was established in 1873 and headed by Newton Winchell, who taught in the winter and conducted surveys during the summer months. Two of his students, young professors from the University of Minnesota, S.F. Peckman and C.W. Hall, spent their vacation in 1879 along the north shore of Lake Superior studying rocks. A report they published in 1888 is the first printed reference to Thomsonite.

During the turn of the century, Hans Bernard Larsen, a Norwegian immigrant who came to Grand Marais in 1888, sold Thomsonite in a summer tent along Good Harbor Bay. By 1924, the American Exploration Company owned much of the Thomsonite-laden property.