It is not a magic item, but a magic-imbued object.
This is a patch of skin from a Stone Dragon.
A ritual, similar to Gentle Repose, has preserved this patch and caused the dragon’s own natural regenerative healing properties to temporarily lie dormant.
Should the stone scales be grafted on to another’s skin successfully. Over time, the stone scales will overtake the skin of it’s host, as the host will become permanently attuned to the Stone Scales. This attunement grants the host resistance to Poison as well as resistance to nonmagical bludgeoning, piercing, and slashing damage. Once per day the host can make use of the dragon’s legendary resistance to automatically pass a failed saving throw.
Should the grafting be unsuccessful, the dragon’s regeneration may go beyond the mere skin and slowly overtake the host. Transforming their body, and mind, into a perfectly regenerated version of the original that provided the scales