Touchstone (assaying tool)

This article is about the assaying tool. For the metaphor, see Touchstone (metaphor). For other uses, see Touchstone (disambiguation).

A touchstone is a small tablet of dark stone such as slate or lydite, used for assaying precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace. Basic requirements for a touchstone: hardness on the mineralogical scale 4.6-6.5, the touchstone must be free of cracks and not react with inorganic acids and their mixtures.

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