Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Adămas

（ἀδάμας). A name given by the ancients to several hard substances, and among the rest possibly to the diamond. Psellus describes adamas as follows: “Its color resembles crystal, and is splendid,” which certainly seems appropriate to the diamond. But Pliny (Pliny H. N. xxxvii. 15), in his account of adamas, has evidently confounded the properties of several different minerals, all of which, by their hardness, received from the Greeks the name ἀδάμας. Thus Hesychius applies the name to steel; Pollux to grains of native gold; and Dionysius Periegetes to what was probably fine crystals of quartz. In fact, the ancients knew diamonds, if at all, only in their unpolished state, by which such epithets as “all-resplendent” would scarcely have been suggested. See Gemma.