Green beryl
Beryl = Brille
The german word “Brille” (eyeglasses) derives from the latin word “beryllium” which was used in medieval times for all clear crystals.
Also the words “brilliant”, “brilliance” or the german word “brillieren” (to shine) have the same root: “beryllium”. Beryl was indeed cut to lenses in medieval times and used to manufacture the first eyeglasses.
The extraction of the mineral variety Emerald can be traced back until the 13th century B.C., which makes Beryl to one of the oldest traded gemstones.