The elusive Lake Superior agate. Rockhounds scour Lake Superior’s shoreline for these rare banded beauties—from the western reaches of the Keweenaw Peninsula to the U.P.’s easternmost Whitefish Point.

With the gales of November rolling in, anyone ready to bundle up in raincoats and waterproof boots may have luck spotting Lake Superior agates and other cool rocks. Late-fall weather already whipped ...