Quartz var. Polyhedral Agate

Rare polyhedral Agate from Brazil, famous for the their angular geometric shapes rather than the more typical agate form of rounded nodules. How these formed is still debated but the general consensus is that they are the result of agate forming in the space between preexisting crystals of other minerals such as gypsum, barite, or anhydrite. These were found in the 1970s and the locality that used to produce them is now under water due to the construction of a dam. This is a good large example in excellent condition.