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A turtle-stone is a little object for protecting people against dark magic. The belief in the power of such talismans was very common, but also wrong.

In the short story "The Witcher", Ostrit was carrying a turtle-stone, thinking it would keep him safe against Geralt's magical abilities:

‘You asked for this, you sorcerer. I’ll kill you. Your tricks won’t help you. I carry a turtle-stone.’
Geralt smiled. The reputation of turtle-stone was as mistaken as it was popular.
— pg(s). 24, "The Witcher", in the collection The Last Wish (UK edition)