Thanks for the info folks! It IS marked 18K on the clasp of the necklace, but I have not bought an acid test kit yet. I don't remember if it was marked on the pendant although I'm sure it was marked which got me interested enough to come back later with my scale. It's part of a jewelry auction that the GW has every week. Bids are pretty low and it's part of a lot that's 90% costume jewelry. I think the name is not THAMO, but THAM, but neither of those mean anything to me. I'll put in a bid that won't cost me much if it's fake. I need to get a test kit soon as possible, so I can stop taking risks!