From RAW:

"Keep your stash fresh with the RAW Hyrdrostone! Made from uncoated natural terracotta clay, the humidifying stone is kiln baked. It was originally developed to keep tobacco fresh on ships during the early days of modern trade.

Allow the stone to soak in water for 5 minutes. Then place it with your tobacco or herbs. It will keep your material fresh for a very long time. It can even bring old dried material back to life and is 100% reusable. Serious game changer brought back from the past!!"