"The polyhedral shape is& due to the quartz
geode forming in a cavity confined by flat outer crystals, possibly
calcite, that eventually dissolved away."
.
Cut & Polish Polyhedral Agate project
Cut & Polish Polyhedral Agate project
Cut & Polish Polyhedral Agate project
Cut & Polish Polyhedral Agate project
Metamorphosis Quartz with beryl (aquamarine) in between
Quartz var. Polyhedral Agate Geode from Rio Grande do Sul, Brazil