The precambrian Sopa Conglomerate is the source for modern alluvial diamonds deposits in the Diamantina (Minas Gerais) region. This old secondary formation is being reworked by the rivers in the area, releasing the diamonds it stores (themselves originated in primary, still unidentified, sources) into modern alluvia. Both the old conglomerate (Fm. Sopa – Brumadinho – Espinhaço Supergroup, Middle Proterozoic) and modern alluvia have been mined for diamonds since the early XVIII century in the region, in the process giving birth to diamond’s Modern Era.

Several outcrops may be visited in the area: the pictures I have included in the https://xmbl.wordpress.com/geology-rocks-formations-and-places/history-churches-diamonds-and-gold-in-minas-gerais-s-joao-del-rey-diamantina-2004/ photo gallery are from Lavrinha (Campo de Sopa – Guinda), a short travel from Diamantina. Many other outcrops are visible in the region.

You may also read the geological site description (in Portuguese, just an English abstract) in an article by Mário Chaves and Ítalo Meneghetti Filho available here: sitio036.