Nero princeps gladiatorum pugnas spectabat in smaragdo.
The princeps Nero viewed the combats of the gladiators in a smaragdus.
— Pliny, Natural History, 37.16.
There are many fantastical stories to be found in Pliny the Elder’s Natural History. Part of the lure of this encyclopedic work is the (often misleading) conviction with which the statesman explored the objects, peoples, and places of the Mediterranean world. In book 37, Pliny discussed various precious stones valued by the Romans, particularly that of smaragdus (Gr. σμάραγδος). It is often translated as “emerald”, but was in fact a category of green stones that included but was not limited to emeralds. Emerald workers were in fact called Σμαραγταριοί after the stone, and ostraka indicate that Egyptian emerald workers even carried messages as they traded (cf. O Did. inv. 329).
As I have explored in earlier posts about the pearl trade, the notion of what is perceived as a “precious” stone is a social construction that can vary wildly from society to society. Roman jewelry tastes began to shift perceptibly after the acquisition of Egypt as a province at the end of the 1st century BCE, particularly because more emeralds could be found from the area that is modern day Ethiopia. The early medieval encyclopedist Isidore of Seville (16.7.1) notes that emeralds were third place in the hearts of mineral-loving Romans, who preferred the pearls and the unio (another type of pearl) before emeralds.
Like his predecessor Pliny, Isidore also mentions the fact that Nero used an emerald to watch fights, right after noting that the stone was soothing to the eyes of gem cutters. Although both literary mentions are a bit ambiguous (and certainly Isidore is known for simply blindly copying from sources such as Pliny), it is possible that Nero used a concave emerald in order to aid his nearsightedness and to take the glare off on a sunny day. Certainly we know that Roman men (particularly soldiers, farmers, and fishermen) and women wore hats in order to protect themselves from the sun, but sunglasses as such did not yet exist.
Although the citation is not definitive proof, many people have cited Nero’s “emerald” as the first sunglasses, despite the fact that it is unclear if 1. It really was a modern emerald that the emperor was using and 2. How Nero would have used it to reflect images for him. What perhaps lends some credence to the story is the fact that Nero’s tutor, Seneca, was an expert in light refraction, mirrors, and optics.
After reading earlier Greek treatises to inform his Naturales Quaestiones (‘Natural Questions’), the stoic philosopher remarked on the use of glass bowls filled with water in order to magnify small print. Certainly there was already a long history of convex lenses in the ancient world that date back to the ancient Near East (c. 2500 BCE), although arguments still abound over how they were used. Sir Arthur Evans is said to have found lenses at the palace at Knossos, and the British Museum contains the famed “Nimrud Lens” (c. 750 BCE), which is a convex lens from the area of modern-day Iraq that may or may not have been used as such.
Lenses in antiquity seem to have been predominantly rock-crystal lenses. Most were plano-convex ground for use in magnifying objects. One such lens was found in the “House of the Engraver” at Pompeii, and it has been proposed that such lenses helped engravers to achieve precision when carving gem stones or creating gold glass (a type of art I have discussed here).
Reflective surfaces used for concentrating rays for burning or those used as weapons were also quite known in Greco-Roman antiquity. The famed “death ray” mirror developed by Archimedes to catch the Roman fleet on fire in 212 BCE has indeed been shown as a feasible way to catch a ship on fire (with little cloud cover), according to an experiment performed at MIT.
Although mirrors and magnifying glasses seem to have been common technology in antiquity, sunglasses do not appear to have been an ancient invention. Even the science behind what would become eyeglasses in the Western world was not fully understood until the 10th-11th century. The optical theory behind them was developed by the famed Islamic scientist Ibn al-Haytham, who was called Alhacen in the medieval West. His work on optics was translated into Latin by the 12th century, and his explanations of lenses would go on to influence Roger Bacon and, later, even Renaissance scientists.
In his exploration of the invention of the telescope, Rolf Willach points out that so-called “reading stones” were indeed used on reliquaries such as the “Big-Bernward Cross” (c. 1150 CE) in order to magnify splinters of the True Cross. Medieval monks began to increasingly place such stones on texts in order to magnify small print during the High Middle Ages, but spectacles were not invented until the 13th century (2010: 95-96). It is only then that optics are fully understood and the technology for grinding lenses to a proper fineness and clarity could be achieved (More info on medieval glasses here, via Erik Kwakkel). In other words, if Nero was watching the games through an emerald, it would have been pretty low resolution.
So then who did invent sunglasses? Well, tinted quartz may have been used in China in the 12th or 13th centuries, but many say that the western world lagged behind and did not get sunglasses until the 18th century. It was then that London optician James Ayscough experimented with tinted lenses. What can be emphasized here is that green is indeed the suitable color for saving one’s eyes from the sun.
Into the 19th century, green spectacles were a popular and oft-referenced device found in the works of Nathaniel Hawthorne and Edgar Allan Poe. Clearly there may be some truth to Pliny’s story of Nero using a green stone of some sort in order to watch gladiatorial games, but I would venture to say that the emperor did not wear them as sunglasses and likely saw the games poorly through their use. Rather, Nero was more likely a nearsighted man desperate to impress and, just possibly, was trying out a secret passed onto him by his preferred jeweler or former tutor.
History From Below 
Very interesting! Thank you.