October 2026 – Agate

For October, we’ve chosen a mineral for you that, while common, is also absolutely beautiful: agate. Agate consists of a vast number of tiny silica fibers. Our two agate slices come from the Brazilian state of Rio Grande do Sul. There are very large deposits of the volcanic rock basalt there, which has very large cavities. In these cavities, beautiful crystals of amethyst or calcite can grow, as well as what are known as agate almonds. In this process, the agate completely fills these cavities and—when cut into slices—reveals these wonderful cross-sections.

Chemical formula: SiO₂

Location: Rio Grande do Sul, Brazil