Formation · Cretaceous
Arapey basalt (Serra Geral)
Early Cretaceous unit, Artigas · Rio Grande do Sul
The Arapey Formation of Artigas, Uruguay, is the local name for the Early Cretaceous Serra Geral flood basalts whose gas cavities host the world's finest amethyst geodes, alongside agate, calcite and, in Brazil's Ametista do Sul district, the largest cathedral geodes on the market. Uruguayan amethyst is prized for deep saturated purple, small crystals and druse; Brazilian geodes run larger and paler. Heat treatment turns pale amethyst into 'citrine' and is common, and cut bases with steel stands are standard; state whether colour is natural, as the Stratum sample record does.
| Country | Uruguay |
|---|---|
| Region | Artigas, Rio Grande do Sul |
| Age | Hauterivian, Early Cretaceous |
| Age range | ~135–130 Ma |
| Group | Artigas district |
| Macrostrat rank | Fm· Macrostrat |
| Macrostrat age | 136–121 Ma· Macrostrat |
| Lithology | basalt, ignimbrite· Macrostrat |
| Environment | non-marine· Macrostrat |
| Macrostrat columns | 8 column(s) sampled from 8 unit(s)· Macrostrat |
Collecting and legality
Amethyst geodes are mined under Uruguayan mining concessions in the Los Catalanes district and Brazilian concessions around Ametista do Sul; export is routine with an invoice and, for Uruguay, a DINAMIGE export declaration. This is a mineral, not a fossil, so no paleontological permits apply.
Lithology and setting
Flood basalts of the Paraná–Etendeka large igneous province; gas cavities in the upper flows filled with quartz, amethyst, calcite and agate.
Elsewhere
Cite this article
Stratum Study. "Arapey basalt (Serra Geral)" (Formation). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/formations/arapey-basalt-serra-geral. Accessed 2026-09-20.
@misc{stratum-arapey-basalt-serra-geral,
title = {Arapey basalt (Serra Geral)},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/formations/arapey-basalt-serra-geral}},
year = {2026},
note = {Formation article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, macrostrat. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.