What Causes Rashes On Chest After Wearing Necklace?

I got a necklace for Christmas and have been wearing it ever since. I started to breakout on one side of my chests last week and alittle in front of neck. Today I have a rash with hives on the other side of my chest today and back of my neck. It itches real bad and hurts some. I took necklace off thinking it might be this. What can I use to get rid of the rashes and itching.