Touch'-stōne, n.
n. 1. (Min.) A variety of extremely compact silicious schist, almost as close as flint, used for ascertaining the purity of gold and silver by the streak impressed on the stone;—called also Lydian-stone and basanite.
Sense 2. Hence, any test or criterion by which the qualities of a thing are tried.
The foregoing doctrine affords us a touch-stone for the trial of spirits. South.
Irish touch-stone (Min.), the basalt, the stone which composes the Giant's Causeway.