Forget the expensive kits that have multiple different vials of acid - get only the 18kt gold testing bottle and you are set to test for both gold from 14k up to 24k or silver from 675 up to 999!
How does it work?
Gold:
For 14k gold the 18k acid will eat away the gold in about 8,9-10 seconds, any other base metals will dissolve almost instantly, the 18k gold will stay and not disappear
Silver:
The magic here is great! All silver purity from about 600 on up will turn a chalky white and not dissolve with this acid and as time progresses and you do more tests - by the whiteness of the chalk left over you will be able to tell the difference s in the metals purity.
The ring in the photo had a little of both 18k gold and 950 silver.
Have fun! Cheap!