Nagastone is the main block that makes up the wall surrounding the Naga Courtyard. It has about the same minability of a normal stone brick. They CAN be broken by the Naga, so don't try to use them for protection.
Nagastone has an interesting behavior. If placed next to another Nagastone, the first one will turn into a head design, while the second one will turn into a body design. Any additional bricks will become body segments.