Thomsonite Beach

One of the rarer zeolites on Earth, and it weathers out of the basalt onto a Lake Superior shore.

Thomsonite forms in the amygdaloidal cavities of basaltic volcanic rock, alongside other zeolites, and occasionally in granitic pegmatites. Central Michigan University's mineral reference illustrates it with polished pieces taken from a basalt outcrop about five miles south of Grand Marais, Minnesota, one of them a spheroid a little over a centimetre across. That is this shoreline, and that is why people walk it with their heads down.

The name is a technicality worth knowing if you buy any. Thomsonite was treated as a single mineral species until 1997, when the International Mineralogical Association reclassified it as a series name covering two species, thomsonite-Ca and thomsonite-Sr. The calcium one is by far the commoner, a hydrous sodium, calcium and aluminium silicate; strontium can substitute for the calcium, and which species you have depends on which element dominates. The two are visually indistinguishable, so unless somebody has actually tested a piece, the series name is the honest label.

It was first identified from Scottish material in 1820 and named for the Scottish chemist Thomas Thomson. It is orthorhombic, sits at 5 to 5.5 on the Mohs scale, has a density of 2.3 to 2.4, and runs from transparent to translucent. Colours are colourless, white or beige, and sometimes green, yellow or red.

The shapes are the appeal. The crystals grow as long thin blades that usually form radial aggregates, and sometimes fans and tufts. Those aggregates vary a great deal: some spikey, some dense and ball-like, some growing in worm-like forms, with tight radiating needle clusters and small spheres among the commonest. Cut and polished across, the radiating structure is what gives the stone its pattern.

It turns up in a scattered list of places, among them the Faroe Islands, Scotland, Arkansas, Colorado, Michigan, New Jersey, Oregon, Ontario, Nova Scotia, Gujarat and the Urals. Very few of them are a beach you can walk.