from
The Collaborative International Dictionary of English v.0.48
Touchstone \Touch"stone`\, n.
1. (Min.) Lydian stone; basanite; -- so called because used
to test the purity of gold and silver by the streak which
is left upon the stone when it is rubbed by the metal. See
{Basanite}.
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2. Fig.: Any test or criterion by which the qualities of a
thing are tried. --Hooker.
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The foregoing doctrine affords us also a touchstone
for the trial of spirits. --South.
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{Irish touchstone} (Min.), basalt, the stone which composes
the Giant's Causeway.
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