Agates are unable to be reproduced in a laboratory environment, and hence the mechanism of their formation remains unknown. Present hypotheses regarding genesis are thus, inevitably, speculative. We describe novel experiments, at room temperature and pressure, producing banded amorphous silica gel (via a sol–gel transition) whose morphology is compared to that of agates. In addition, macro- and microscopic examination of agates from several sites revealed some distinctive physical features: surface pitting, indentations of the agate bands, the variation in morphologies of the bands, thinning and narrowing of the bands around the indentations and the escape tubes, shapes of the coloured impurities and persistence of the sharp angle of the bands at sector boundaries. These observations and experimental data lead us to conclude that agates can be formed in four stages: sol–gel transition with concomitant band formation, formation of escape tubes, rapid crystallization from multiple nucleation sites and late dissolution/recrystallization of the inner layers.

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