4 New Acid Test Needles. This is a new set of 4 testing needles. These needles test 10K, 14K, 18K gold and silver. They are used to determine karat content of gold and silver. The needles and the item to be tested are rubbed on a test stone and treated with acid to determine a match. FindingKing works hard to update our descriptions and pictures of our merchandise. As part of a process of ongoing innovation by our suppliers, newer versions of the item shown may be shipped before we are able to complete an update. As a result there may be a slight variation in color and design.