Prior to 1997, thomsonite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named thomsonite-Ca and thomsonite-Sr. Thomsonite-Ca, by far the more common of the two, is a hydrous sodium, calcium and aluminium silicate, NaCa2Al5Si5O20·6H2O.

The crystals tend to be long thin blades that typically form radial aggregates, and sometimes fans and tufts.

Thomsonite occurs with other zeolites in the amygdaloidal cavities of basaltic volcanic rocks, and occasionally in granitic pegmatites.

Faroelite[2]), Scotland, Arkansas, Colorado, Michigan, Minnesota, New Jersey, Oregon, Ontario, Nova Scotia, India, and Russia.

[3] Nodules of massive thomsonite that display an attractive banded coloring are found along the shore of Lake Superior.