This study documents a novel form of ferric iron oxyhydroxide-rich moss agate, taken from the flanks of the TAG (Trans-Atlantic Geotraverse) submarine hydrothermal mound at 26°N Mid-Atlantic Ridge. The genesis of the agate is related to a series of rapid irreversible changes, with cooling across a redox and pH front separating oxidizing highly viscous siliceous gels from a mixed pyrite–iron oxide sediment. This study shows a fraction of the inorganic self-organized mechanisms of mineralogical, textural, and geochemical patterning operative within these far from equilibrium settings, and how iron inclusion morphologies suggestive of a biogenic origin may be generated by inorganic processes.

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