Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
glaesum (glēsum, glessum), i, n. [cf. Germ. Glas; gleissen = glänzen, to glimmer; Engl. glass], amber, Tac. G. 45; Plin. 37, 3, 11, § 42.
Hence, glaesārĭus, a, um, adj., of amber, amber-: insula, which produces amber, Plin. 4, 13, 27, § 97; 37, 3, 11, § 42.