Go to brambleclan and keep going until you see a diamond arrow and press it. There will be two caves. The coldstone is in the left one. But the only way i know how to get in is to kill all the cats guarding it.
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you can ask lunarclaw to be her apprentice, then follow her instructions and heal cats untill she says 'youre an exp medicine cat' when you go into her den, usually it'll take like 10 cats healed, you can also used the comand magic (i belive, if not, try wizard) to automatically get the herbs needed to heal a cat