The volcanic glass of Vvardenfel is not the only type of glass to be found on Tamriel. In fact, the green glass used by the Chimer/Dunmer may be a poor substitute for the types of glass that were available to them before Veloth led them away on their exodus to far off Resdayne. After all, the green volcanic glass does not seem to be intrinsically magical.
The glass available to the High Elves in the Summerset Isles comes from meteoric and possibly synthetic sources. Meteoric glass includes Welkynd, Varla, Malondo and Culanda. As we can see in the game, large quantities of raw, unworked meteoric glass can be found underground in many locations.
The Culanda Lacquer and Vitrified Malondo (see, it's even called "vitrified") that are used for Psijiic and Sapiarch armour may be very highly processed or even magically synthesised.