TOUCH-STONE Copy link • TOUCH-STONEPROP., coticula: lapis Lydius (Plin.); “schistus Lydius (Linn.). Figuratively, obrussa alicujus rei (Cic., Sen.); lex, norma, qua spectetur, ad quam exigatur, aliquid. To apply a touch-stone to anything, aliquid ad obrussam exigere (Sen.): this is a touch-stone for anything, haec est alicujus rei obrussa (Sen.).