Toothpaste might work. However I have A LOT of jewellery and have become very friendly with our jeweller.

Here is the deal! And they use this in the shops!

GOLD:::: Ammonia & washing detergent with some hot water. Soak for 30 secs then rinse. Or an easier option - window cleaner and washing detergent!

SILVER::::: Washing soda (NOT washing powder) with alfoil. Ok, you put the alfoil in the bottom of a bowl. Add 1 tablespoon of washing soda. Fill with boiling water then add the item to the mixture ensuring it touches the alfoill. This method works for silver and gold, but the one above works much better.