E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A dark, flinty schist, called by the ancients Lapis Lydius; called touchstone because gold is tried by it, thus: A series of needles are formed (1) of pure gold; (2) of 23 gold and 1 copper; (3) of 22 gold and 2 copper, and so on. The assayer selects one of these and rubs it on the touchstone, when it leaves a reddish mark in proportion to the quantity of copper alloy. Dr. Ure says: In such small work as cannot be assayed the assayers . . . ascertain its quality by touch. They then compare the colour left behind, and form their judgment accordingly.
The fable is, that Battus saw Mercury steal Apollos oxen, and Mercury gave him a cow to secure his silence on the theft. Mercury, distrustful of the man, changed himself into a peasant, and offered Battus a cow and an ox if he would tell him the secret. Battus, caught in the trap, told the secret, and Mercury changed him into a touchstone. (Ovid: Metamorphoses, ii.)
Gold is tried by the touchstone, and men by gold.Bacon.
Touchstone. A clown whose mouth is filled with quips and cranks and witty repartees. (Shakespeare: As You Like It.) The original one was Tarlton.