a touchstone , test-stone , a very hard stone , used as a test for the precious metals , for whetstones , and , in medicine , for mortars , Plin. 36, 20, 38, § 147; 36, 22, 43, § 157; Isid. Orig. 16, 4, 36.—Also basanites alone, Plin. 36, 7, 11, § 58.