Beautiful amethyst geode with an agate rim

An amethyst geode from Brazil, whose outer edge bears a striking agate rim. The sequence from the agate base to the amethyst tips can be read as a complete geode genesis narrative.

Agate and amethyst are both varieties of quartz and formed in the same geode in temporal sequence. Agate first crystallizes from the finely dispersed silicic acid-containing solutions at the geode's edge, forming the typical concentric bands. Only then does the inner volume crystallize with larger amethyst crystals.

Such complete sequence geodes are didactically valuable: they show the original outer cavity wall (agate bands) and the lining of the inner chamber (amethyst tips) in a single specimen. A geological teaching aid in stone.

Brazilian geodes from Rio Grande do Sul regularly provide such complete stratigraphic specimens in collector quality. They are recovered from the basalt flows of the Serra Geral Formation.

A collector's item with complete geode genesis readability.