This is a classic example of iridescent Goethite [FeO(OH)], sometimes called “turgite” in the trade, from the well-known Filón Sur pit at the Tharsis mines in Huelva, Spain.
The specimen is built of stacked, rounded botryoidal globules that have grown into several elongated, finger-like or stalactitic columns. Its base coloration is dark steel-gray to brownish-black, with a metallic to sub-metallic luster. A thin surface film interferes with light, causing some light to reflect off the top surface and some to reflect off the bottom surface, resulting in vivid iridescence that reveals magenta, fuchsia, electric greens, golds, yellows, blues, and purples across the specimen.
A fantastic display piece, the combination of morphology and rainbow sheen is exactly why this material is so popular with collectors.
