Thomsonite

Thomsonite is a calcium-sodium zeolite — a hydrated aluminosilicate — that forms in the gas cavities of basalt alongside other zeolites. Collectors know it best as small, rounded nodules with concentric, eye-like banding in green, pink, white and black.

Identifying thomsonite

Thomsonite is moderately hard (5–5.5) and usually occurs as radiating, fibrous masses and spherical aggregates rather than distinct crystals. Cut and polished, its banded nodules show the 'thomsonite eyes' that make it sought after.

Where it is found

The most famous source is the Lake Superior region of Minnesota and Michigan, where thomsonite pebbles weather out of the basalt and are gathered along the shore; it also occurs in basalt zeolite localities worldwide.

For collectors and lapidaries

Tight, colourful concentric 'eyes' with good contrast and a clean polish are the most prized, usually cut as cabochons.