In this video I have the opportunity to cut polyhedroid agates from Paraiba, Brazil.
These agates have all been mined in the 70s, so material is near impossible to find. You might find some white ones, which have a higher chance of being quartz, but black skinned nodules are almost non existent anymore and very rare to find.
As far as I know, this is the only video on youtube of cutting Polyhedroid agates!
Enjoy.
VHAgates