My wife uses these on all her silver jewelry with semi-precious stones as she has found that liquid silver cleaner damages the stones.  Wish they weren't so expensive, but they definitely work.  Subtracted one star for expense, not effectiveness.

Update:  Thought I would add that our local Wal-Mart has these for $5.88, but they are often out-of-stock.  Strangely they are $9 each if you order it on the Walmart website.