Quartz pseudomorph after Calcite - Rio Grande do Sul, Brazil
 
Quartz geode with a 3 cm encasing pseudomorph from quartz to calcite. The calcite crystal has dissolved leaving a quartz shell in calcite shape (perimorphosis). When cracking open or sawing open geodes, it is precisely these types of rarities that are hoped for.
A mineralogically interesting, extremely aesthetic and rare collector's item from Brazil.