My friend wont return an item I lended to her?

I let her borrow my gold necklace because I always do and I trusted her but it was the last day of school and I didn't really notice so she kept it all thru summer and promised me she would give it back next year but we ended up going to different schools and I asked for it thru facebook and she said "I don't want to it's the only thing that reminds me of you" and btw we went out but i know she didn't really care about me lol...so anyways is there anything I can do to get it back? I don't see her anymore and what am i supposed to tell thee police if that were the last option..can anyone help please that necklace was my grandpas and he recently passed.