The Lycurgus Cup is a Roman-era glass goblet that changes color from jade green to ruby red depending on if a light is shining on its front or back. But experts are unsure where the cup was found, where it was made, or whether the glassblower who made it created the color-changing effect on purpose.

In 1958, the British Museum acquired the cup from the Rothschild banking family, which had owned the artifact since the mid-19th century. Known colloquially as the Lycurgus Cup for its depiction of the mythical king who angered the god Dionysus, the glass is about 6.3 inches (15.9 centimeters) tall and weighs around 1.6 pounds (700 grams).

The glass is a remarkably well-preserved example of a Roman "cage-cup," according to a 1959 study of the artifact by archaeologists D. B. Harden and Jocelyn Toynbee. Only a handful of cage-cups still exist, and experts believe the goblets were made by cutting and grinding decoration into thick glass. Because small bridges were left to ensure the decoration remained on the glass, the overall effect is like a cage. Harden and Toynbee estimated that the Lycurgus Cup was manufactured in the fourth century, likely in Italy, but they noted that the bronze frame that forms the stem and rim of the goblet is probably a 19th-century addition.

Depicted on the cup is the myth of the punishment of Lycurgus, king of the Thracian tribe called the Edoni (also called the Edonians). Lycurgus did not approve of the worship of Dionysus and banished the god into the sea with an ax. But one of Dionysus' priestesses, Ambrosia, decided to fight Lycurgus. Ambrosia changed herself into a vine and wound herself around Lycurgus, who was unable to escape. The other priestesses thronged around them and tormented Lycurgus. Given that Dionysus was a god of wine and merriment, the depiction of Lycurgus' punishment may have been particularly appropriate for this goblet. Additionally, the change in color from green to red may have been meant to mimic the ripening of grapes on the vine.

Color-changing cups, also called dichroic cups, were mentioned in ancient Roman historical records. For example, a second-century letter possibly penned by Emperor Hadrian to his brother-in-law mentions a gift of two dichroic cups to use during banquets. But very few fragments of this mysterious type of glass survive today, a 2007 study noted.

For decades, experts have been working out how ancient Romans produced dichroic cups, and some have even replicated the Lycurgus Cup's color-changing effect. A 1963 study confirmed that the key seems to be the addition of microscopic nanoparticles of gold and silver during glassblowing that produce the appearance of milky-green glass when lit from the front and nearly translucent red glass when lit from behind. The metal nanoparticles are smaller than grains of sand and interact with light, creating the perception of a color change.

A replica of dichroic glass made by Corning Glassworks in the 1960s went from green to amethyst in the light, the 2007 study reported, and in 2020, researchers included silver and gold nanoparticles in 3D printing to replicate the color-changing properties of the glass.

More recently, experts at the University of Sheffield in the U.K. reproduced the dichroic glass by reverse-engineering the "recipe" from the original Lycurgus Cup. The team then commissioned a glass artist to create a Lycurgus-inspired design from the replica.

But because so few examples of dichroic glass exist, experts are still unsure whether Roman artisans created the color-changing effect on purpose or whether traces of gold, which would have been available in an ancient craft studio, slipped into the molten glass and made a rare and unusual material that mystified even the most skilled glassblowers.

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