This article's content is accurate to the most recent public game version, 0.4.8d.
"Smash it to release a body-purifying burst of magic."
Seals of Purity are consumable items that remove the 
Poisoned status effect from one ally.
Some skills such as Natalie's Cleansing Light are also capable of performing this, and for a fairly cheap MP cost too. Lume-wise it would be more efficient to just buy MP recovery items to top up Natalie. For mid-battle recovery if Natalie is otherwise occupied it is not too bad of an idea to carry several of these on hand, though if you have the time farming a few Honeys may be better as those do the same thing along with restoring some HP.
The following merchants sell Seals of Purity:
The following monsters drop Seals of Purity:
- Kobold (25% chance)