The Lycurgus Cup is a cage cup — the whole decoration was ground back out of a thick blank of glass, leaving a lattice of figures hanging free. That part is insane on its own: undercutting glass so the figures stand proud without breaking. But the cage is not what makes it famous.
The glass itself is dichroic. Seen in reflected light it is green. Hold a lamp behind it and it glows red. For most of the twentieth century this was a standing mystery in the British Museum. People could not even agree on what it was made of — until 1950 it was genuinely uncertain whether the cup was glass at all or some carved gemstone.
In 1990, researchers put a fragment under a transmission electron microscope and found nanoparticles of a gold-silver alloy, roughly 70 nanometres across, suspended through the glass. At that size the particles are smaller than the wavelength of visible light, and they interact with it in a way bulk metal never could — a surface plasmon resonance. The exact physics that powers modern nanoscience, sitting in a 4th-century drinking cup.
Here is the part that makes it cursed: the Romans could not have done this on purpose. The quantities are 40 parts per million of gold and 330 of silver — levels you cannot measure, let alone dose, with any technology of the era. No optical microscope could see the particles. The most likely explanation is contamination: a trace of gold in the silver, or gold dust left on the tools from another job, diluted into a big melt of glass. The cup is a one-in-a-thousand accident.
The strangest detail: one patch of the glass around Lycurgus's torso came out a slightly different colour — an accident of manufacture. The glass-cutter noticed, and exploited it, working the design so the king's rage glows even more strongly than the rest of the cup. They didn't understand the material, but they understood the effect well enough to turn a manufacturing error into characterisation.
The scene itself is the myth of Lycurgus, the king who attacked a follower of Dionysus, was tangled in a vine that grew from her, and died. It may have been read as a dig at a real emperor — Licinius, strangled with a rope in 325, whose defeat by Constantine is sometimes read into the strangling king.
The recipe — accidental or not — only seems to have been used for about a century, and then the knowledge vanished completely. Only fragments of other dichroic Roman glass survive, none with this colour change, none complete. A 4th-century emperor's letter quoted in the Historia Augusta mentions "particoloured cups that change colour" — proof that the effect was prized at banquets — and that is about the whole record.
Corning Glass Works eventually reproduced the effect in the 20th century once they understood the mechanism. Researchers in the Netherlands have even 3D-printed the colour change into a nanocomposite. But nobody has ever worked out exactly how the original was made, and the original makers never knew what they had done.