*Celia, addressing Touchstone, in Act 1 Scene 2 of Shakespeare’s “As You Like It” (1599).
Patricia T. O’Conner and Stewart Kellerman wrote at grammarphobia.com on 1.8.18:
“Long before acids were used in the assaying process, jewelers and others used their own eyes to examine the marks made by precious metals on touchstones.
Because of its ancient connection with gold, the rock known as a “touchstone” had a fascinating history even before it got its English name.
The word is thought to come from a 14th-century Middle French term, pierre de touche (literally “touch stone”), which the Oxford English Dictionary says was first recorded sometime before 1389.
A pierre de touche was (and still is) a piece of stone, typically black jasper or basalt, used in testing the purity of gold and other valuable metals. Similar Middle French terms of the 1400s included touchepiarre and pierre à toucher.
The French began using pierre de touche figuratively in 1579, Oxford says, and around that same time the term also appeared in Spanish as piedra de toque (“touch stone”) in both “concrete and figurative senses.”
However, the practice of testing gold on such a stone preceded those French and Spanish terms by many centuries.
The first-century Roman philosopher Pliny the Elder, in his encyclopedic Naturalis Historia, referred to this stone as coticula (Latin for both “touchstone” and “whetstone”). In Book 33, devoted to metals, he writes:
“Auri argentique mentionem comitatur lapis, quem coticulam appellant. … His coticulis periti cum e vena ut lima rapuerunt experimento ramentum, protinus dicunt quantum auri sit in ea, quantum argenti vel aeris.”
(“A description of gold and silver is necessarily accompanied by that of the stone known as the touchstone. … Persons of experience in these matters, when they have scraped a particle off the ore with this stone, as with a file, can tell in a moment the proportion of gold there is in it, how much silver, or how much copper.”)
The use of similar stones was also known in ancient India, Egypt, and Greece, according to historians and metallurgists.
But let’s get back to English and the word “touchstone.”
In early uses, it was also spelled “twichstone,” “touche stone,” “towtchstone,” “tuitchstone,” and “tweichstaine.”
Here’s the OED’s earliest example of the literal usage: “Touche stone to proue golde with.” (From John Palsgrave’s Lesclarcissement de la Langue Francoyse, a 1530 French grammar book written in English.)
Oxford defines “touchstone” as “Fine-grained black stone (typically a type of chert) upon which objects made of gold or silver can be rubbed to determine their purity; a piece of this.” (The cherts are silica stones like flint, jasper, agate, onyx and others.)
As the OED explains the process, “the touchstone was originally used in conjunction with a set of touch needles of known purity, allowing visual comparison of the mark left on the stone by the object being assayed with those of the touch needles.”
The word must have been known before it was recorded in writing in 1530, since the figurative use appeared in the same year. Here’s the earliest example we’ve found:
“Ye scripture is y twichstone yt tryeth all doctrynes” (“The scripture is the touchstone that tests all doctines”). From William Tyndale’s 1530 translation of the Pentateuch.
The earliest figurative example given in the OED is from John Frith’s A Disputacio[n] of Purgatorye (1531). Here Frith invites critics to test his arguments by consulting scripture: “Laye them to the touchstone and trye them with goddes worde.”
(In the treatise, Frith uses “touchstone” figuratively four times altogether. In another passage he says that the “worde of god” is the “perfeyte touchstone that iudgeth and examineth all things.”)
The OED defines the figurative sense of “touchstone” as “Anything which serves to test the genuineness or value of anything; a test, a trial; a criterion or reference point by which something is assessed, judged, or recognized.”
We should mention here that “touchstone” at one time had another meaning, one probably derived from the gold-testing term. It meant a “fine-grained dark stone used for building and monumental work; esp. a type of black marble,” the OED says.
In a chronological oddity, this use was found in writing in the 1480s, decades before the parent term. As Oxford explains, “in spite of the chronology of the examples, it is likely” that the use of “touchstone” to mean black marble developed from an earlier metallurgical sense—the black stone used to test gold.
As more old manuscripts are digitized and made available to scholars, earlier uses of “touchstone” may come to light.
Today, English speakers still use “touchstone”—and French speakers still use pierre de touche—in both literal and figurative ways.
The French-English online dictionary Linguee gives this figurative example: “Le livre était considéré comme la pierre de touche du genre fantastique” (“The book was considered the touchstone of the fantasy genre”).
And the OED has modern English examples for the gold-testing term as well as the figure of speech. Here’s a sampling:
“In a metals shop the most common method for determining the karat of gold is with the use of a touchstone.” (From The Complete Book of Jewelry Making, 2006, by Carles Codina, translated from Spanish by Laura C. Jones.)
“Fashion, literature and music are the cultural touchstones by which we navigate our recent history.” (From the online London newspaper City A.M., June 4, 2015.)
Finally, this one from the March 2012 issue of Vanity Fair uses the figurative word adjectivally. It’s from an article about the 1982 comedy Diner:
“For a certain 40-plus demographic … the movie became … a touchstone experience, its lines serving as passwords, signifiers of like-mindedness.”.”