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Obsĭus, i, m., a Roman surname, Plin. 36, 26, 67, § 196.
Hence, Obsĭānus, a, um, adj., belonging to Obsius: lapis, so named after the discoverer, Plin. 37, 10, 65, § 177.
Also subst.: obsĭāna, ae, f., Plin. 37, 13, 76, § 200; and obsĭāna, ōrum, plur. n., a kind of glass: obsiana numerantur, id. 36, 26, 67, § 196. (The older editions of Pliny read Obsidius and Obsidianus; hence the name obsidian applied to the stone.)