touchstone

noun

1. a test or criterion for the qualities of a thing.
2. a black siliceous stone formerly used to test the purity of gold and silver by the color of the streak produced on it by rubbing it with either metal.

noun

1. a criterion or standard by which judgment is made
2. a hard dark siliceous stone, such as basalt or jasper, that is used to test the quality of gold and silver from the colour of the streak they produce on it

n.

late 15c., from touch (v.) + stone (n.). Black quartz, used for testing the quality of gold and silver alloys by the color of the streak made by rubbing them on it. Cf. also basalt. Figurative sense is from 1530s.