To Have and to Hold - Ruth Skrine - Bog

Celia, an isolated woman in her mid-twenties, finds an abandoned baby with a metal chain round his neck. She doesn't return to the commune where she has been living but seeks shelter in a farmer's barn. He is kind and in return she leaves the chain for him.
He discovers it is real silver and sets out on a quest to return it. Apparently benign, the necklace reveals unexpected evil powers.