Middle Primitive Elvish
khel
root. freeze
khelek
root. ice
khyel(es)
root. glass
khyelesē
noun. glass
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A root in The Etymologies of the 1930s glossed “glass” with derivatives in both Quenya and Noldorin with the same meaning: ᴹQ. hyelle and N. hele (Ety/KHYEL(ES)). After Tolkien changed Noldorin to Sindarin, he decided that “There was no common Eldarin word for glass”, and that the Sindarin word S. heledh was derived from Khuzdul kheled (PE17/37). Thus the root ᴹ√KHYEL(ES) was abandoned.