According to the records of Pliny the Elder, Roman Emperor Nero during gladiatorial fights looked at the arena through a concave emerald. In this way, he eliminated the glare of the sun and his nearsightedness.
It is believed that those may have been the first sunglasses in history. Ancient Romans valued emeralds (smaragdus), green gemstones.
It is worth mentioning that the ancient Romans, the same as the people of the Middle Ages, were not able to precisely distinguish some minerals, and therefore they generally called all green beautiful stones emeralds, red rubies, and blue sapphires. Although ancient sources do mention Nero’s emerald, today the opinion is that Nero actually used another transparent stone with a beautiful green colour, such as olivine.
Pliny the Elder in the “Natural History” indicates that there are 12 types of emeralds, and at the same time indicates that they stand out from the Egyptian and Scythian ones because they cannot be scratched. Real emerald (beryllium variety) has a very high hardness factor (close to 8 on the Mohs scale), so it is indeed difficult to scratch. This would mean that the other 10 varieties of emeralds indicated by Pliny are other green gemstones of lower hardness – e.g. green chrysolites (olivine) or tourmalines. Thus, the word “emerald” used by Pliny to refer to Nero’s “eyepiece” cannot be read strictly according to our present-day criteria, but rather broadly – simply as a transparent green stone.