Thomsonite

Thomsonite occurs with other zeolites in the amygdoloidal cavities of basaltic volcanic rocks, and occasionally in granitic pegmatites. Examples have been found in Faroe Islands as "Faroelite" and in Scotland, Colorado, New Jersey, Oregon, Nova Scotia, India and Russia.

Nodules of massive Thomsonite that display an attractive banded coloring are found along the shore of Lake Superior.  Most of these Thomsonite nodules and their derived pebbles are less than 0.6 cm (1/4 inch).  Those enclosed in basalt are extremely difficult to remove without breaking them.  These specimens must be carefully and patiently removed from the stone using diamond carving tools specifically made for gemstone design.

Thomsonite from the Lake Superior region is considered to be one of the largest types of zeolites.  It has a hardness of 5.5 and a colorless streak.  Gemstones larger than a half inch are considered exceptionally rare in a polished and finished piece.