Amethyst Geode with Huge Twin Calcite Crystal

An amethyst geode from Uruguay with a prominent honey-yellow calcite twin crystal. It protrudes from the interior of the geode like a central plinth element.

Calcite twins form when two crystal axes grow from a common root point along different axes. They mirror each other on a common plane of symmetry. This growth habit is a rare mineralization specialty.

The amethyst geode itself shows the uniform violet coloration typical of Uruguayan material, with a light agate rim. The tips are fine and densely distributed.

What makes this geode special is the combination of amethyst and calcite twin: two mineral growth forms in one geode. The calcite crystallized as a later phase from residual solutions, after the amethyst growth.

The warm honey tone of the calcite is due to iron trace elements in the carbonate lattice. A collector's item with didactic value, ideal as a solitaire in a collection.