Yooperlite

Yooperlite is the trade name for pieces of sodalite-rich syenite that fluoresce a brilliant orange under longwave ultraviolet light. By day they look like ordinary grey speckled stones; under a UV lamp the sodalite within them lights up in glowing orange and yellow veins. They are a rock containing fluorescent sodalite, not a single mineral species.

What makes them glow

The fluorescence comes from sodalite in the rock, activated by disulfide ions built into its structure. Yooperlites are best hunted after dark with a 365-nanometre UV flashlight, when the glowing sodalite stands out instantly against the dark beach.

Where they are found

Yooperlites are famous from the shores of Lake Superior in Michigan's Upper Peninsula — the name comes from 'Yooper,' a nickname for residents of the U.P. They were identified and named by Erik Rintamaki in 2017. The stones are glacial erratics, carried south from large alkaline igneous complexes in Ontario, Canada, then rounded by the waves of Lake Superior; the richest hunting grounds lie between Muskallonge Lake and Whitefish Point.

For collectors

Pieces with broad, bright orange fluorescent veining are the most desirable, whether left natural as beach cobbles or polished to show the glowing sodalite. A UV light is essential to appreciate them.