Polyhedral Agate.  
Peekaboo!
I have been working energetically with this piece and admiring the beauty.  Time to pass on....
2 sides with a great polish...my friend who I get these from cuts and polishes them to a high standard. He is a diamond merchant and knows his stuff!  
 
A very rare variety of agate/quartz.
Found in the mid 1970s in Paraiba, Brazil
It is thought that they formed in the cavities between large crystals of angel wing calcite.
These have not been mined since the 70's, so pretty hard to come across.
 
This beautiful specimen weighs a whopping 338 grams in weight and 
measures 13cm x12cm x 9cm and width is just over 3cm