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• Natural assay stone The touchstone is a fine-grained black Lydian that has proven itself to be the best material because it possesses all the characteristics required of a good testing stone. It must be uniformly matte black, has a homogeneous, non-porous structure and is harder than the metals being tested, it is resistant to acids and the stone may not be worse of quality after a few cleanings. Despite many years of attempts, it has not yet proven to develop an artificial material with these ideal characteristics. VanDijk Toetsstenen touchstones are in various sizes, and so also in weight, this is because only the best part of the original rough stone can be used. The stones become, after sawing, a long-standing polish process. After polish the stone will be totally checked at imperfection so that finally every stone has the same quality.