Glasses can become non-vegan through animal-derived glues (like gelatin-based ones), certain dyes (carmine from insects), or finishes, but primarily through hidden animal products in things used with glasses, like non-vegan cleaning solutions, or even animal-derived materials in the lenses themselves or frames if they aren't cellulose acetate or metal/titanium. Always check labels for certifications or ask companies about ingredients like gelatin, carmine, shellac, or animal hair/leather accents in frames or coatings.