I lost my new friend's valuable necklace, what should i do?

i lost one of my friend's necklaces that he lent to me, i had no idea the ring attached to it was a family heirloom, if i had known i wouldn't have even accepted it being lent to me. well the point is that i lost it, i'm afraid to tell him. i've only known him for like a month so i dunno if he will be able to forgive me, if he's a true enough friend yet. you think he'll be so mad he won't speak to me again or worse?