ASA Digital Art’s hypothesis: after the fragments settled, the land that held them rose and became desert. We have not matched this to a dated, sourced desert episode in the São Francisco–Congo region, and so we give no date.
What the record does establish is that carbonado-bearing sediments were repeatedly reworked. In Brazil, carbonado is recovered from conglomerates—such as the Tombador conglomerate of the Chapada Diamantina in Bahia—that were reworked over a span from at least 1.7 to about 3.8 billion years ago[1]. Today most stones are won from alluvial gravels[1,2].
Reworking of that kind implies cycles of uplift, erosion and redeposition, but it does not specify a desert. Our hypothesis is offered as a reason to look, not as a reconstruction.
The next essay, on a possible varnished surface, explains why the desert idea matters to our own observations.