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touchstone

   Also found in: Dictionary/thesaurus, Legal, Acronyms, Wikipedia, Hutchinson 0.01 sec.

touchstone

Black, silica-containing stone used in assaying to determine the purity of gold and silver. The metal to be assayed is rubbed on the touchstone, and then a sample of metal of known purity is rubbed on the stone right next to it. The streaks of metal on the stone are treated with nitric acid, which dissolves impurities, thus increasing the contrast between the two samples when compared. Because other metals, such as copper, can be alloyed to silver without changing its colour significantly, the touchstone method is not usually used now to assay silver, though it is still used to assay gold and provides a reasonably accurate guide to quality.


touchstone
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

Touchstone
a “motley-mined,” “roynish” court jester. [Br. Lit.: As You Like It]
See : Clowns


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? Mentioned in ? References in classic literature
 
But the touchstone of the New Learning was the knowledge of Greek, which had been to the greater part of Europe a lost tongue.
Whether his religion was real, or consisted only in appearance, I shall not presume to say, as I am not possessed of any touchstone which can distinguish the true from the false.
The woman took the thimble and examined it, weighed it, and submitted its metal to the test of the touchstone.
 
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