Black stones are used for determining the karat of gold. Gold is tested by being scratched onto the surface of the stone. The stone is to be paired with testing solution/acid to accurately determine the karat content of the scratch that was made. Measures 3" x 2" x 1/2". Great quality and economical! Essential for Jewelry Repair and Jewelry Making. Please contact us with any questions. Please visit our store for other great jewelry tooling items such as; Hand Stamps, Disc Cutters, Polishing Compound, Buffers, Hammers, Tweezers, Mandrels, Anvils, Vises and more! Thanks for Looking!