From Proto-Germanic *glasą, originally meaning 'amber' — transferred to the artificial material, both united by PIE *ǵʰel- (to shine).
A hard, brittle, typically transparent substance made by fusing sand with soda and lime.
From Old English "glæs" (glass), from Proto-Germanic *glasą (glass, amber, resin), likely from PIE *ǵʰel- (to shine, to gleam, yellow, green). The PIE root *ǵʰel- is one of the great colour/light roots, producing an extraordinary range of descendants: "gold" (from Proto-Germanic *gulþą), "yellow" (from *ǵʰelh₃-wo-), "glow," "gleam," "glitter," "glint," "glimmer," "glare," and "gloss" — a remarkable cluster of gl- words denoting light effects in English. Through Latin, the root gave "helvus" (honey-yellow) and possibly "holus" (vegetable, greenery). Through Greek, it produced "χλωρός" (khlōrós, green
The Proto-Germanic word *glasą originally referred to amber — the translucent golden tree resin — before being transferred to the new artificial material. When the Germanic peoples first encountered manufactured glass, they named it after the natural substance it most resembled: the gleaming, see-through amber they already knew.