Gnomish
celc
noun. glass, a glass
celchin
adjective. of glass
celchwed
adjective. glassy
celeg
noun. glass
talp
noun. glass
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
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”.