The touchstone method is most common by far and does not damage the item in question. A rubbing of the item is made on a special stone, treated with acids and the result is compared to the result of the same process done on a sample of gold with a known purity. Red radiolarian chert or black siliceous slate were used for this. Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test, using acids and gold samples both of a specific, known concentration.