Pyrite.

Pyrite is iron disulphide, one of the few minerals that crystallises naturally into nearly perfect cubes. The Spanish Navajun deposit produces the famous floater clusters of crisp metallic cubes set in marl, often without the polishing or trimming that most mineral specimens require. The bright brassy lustre and high specific gravity gave the stone the lasting nickname fool's gold, which has clung to it across centuries despite the obvious hardness and crystal-form differences from real gold.

Pyrite has an industrial history as well as a decorative one. It served as the original source of sulphur for sulphuric acid in the nineteenth century, and the strike-a-light flint pairing of older firearms used pyrite or marcasite for the spark.

In modern crystal practice pyrite is paired with the solar plexus chakra and treated as a quiet warmth stone, more useful for steady confident action than for the breathless abundance claims that sometimes get attached to it. The piece is honest enough to recommend on its own terms. Care is the one real consideration: pyrite slowly oxidises in humid conditions, producing iron sulphate (the so-called pyrite decay) that crumbles fine specimens over years. Keep pyrite dry, away from bathrooms, and in dry storage rather than sealed plastic bags that can trap moisture.