Overview

• The residues were identified on unknapped grinding pebbles from Dzudzuana Cave in the Caucasus, excavated from a layer dated to about 34,000 years ago.
• Optical and confocal microscopy revealed blue, sometimes fibrous, residues concentrated in worn zones alongside starch grains.
• Raman and FTIR spectroscopy confirmed the indigotin chromophore, marking the first detection of this compound on artifacts of such antiquity.
• Synchrotron micro-CT showed pore structures capable of trapping micrometric plant remains, explaining residue preservation on the stones.
• Replicative experiments using Nikrisi River pebbles and cultivated Isatis tinctoria produced reference materials, while the plant’s ultimate use—colorant, medicinal, or other—remains unresolved.