|“Which is why it is sealed in this case made of Clearstone, the hardest substance known to human. Impenetrable by any living being.”|
|— Dr. Saunders to the ninja and Wu, "Day of the Departed"|
Clearstone is the hardest substance known to man.
When the ninja try to get a Scroll of Forbidden Spinjitzu, they can't open the case. Cole notes that it's probably Clearstone, but that's not clear, because Nya found that the case could be easily opened.
- Ghosts are known to be able to go through Clearstone.