The Gnomish Lexicon of the 1910s had several glass-related adjectives: G. {celcin} > celchin “of glass”, G. {celcwed} > celchwed “glassy”, and G. celegrin (archaic cailegrin) “glassy, like glass”, all based on the nouns G. celc or celeg/caileg “glass” (GL/25).

Neo-Sindarin: For purposes of Neo-Sindarin I would update these to an adjective ᴺS. heledhren “glassy, like glass, of glass” based on the later Sindarin word heledh for “glass”.