Uruguay is one of the world's main sources of amethyst, and La Oriental is a working mine that lets you walk into it. The guided visit goes underground to the seams, explains how the geodes formed in ancient basalt lava flows, and ends where you would expect, among the cut stones.
They have built the site out as a "campus natural" with walking trails above ground as well, so it works as a half-day rather than a twenty-minute stop. It is inland from Piriápolis near Pan de Azúcar, on Ruta 60.
This is not a tourist mock-up — it is an operating mine with a visitor programme bolted on, which is what makes it interesting. Book by WhatsApp.