The Lycurgus Cup is a 4th-century Roman cup made out of dichroic (two colored) glass, which shows one of the most amazing examples of nanotechnology in the ancient times. The cup looks green when the light comes form outside. However, it looks red, and King Lycurgus looks purple when it is lit from inside.
In 1990, scientists analyized the glass to explain the situation. According to the results, the dichorsim results from the presence of nanoparticles that are silver (66.2%), gold (31.2%), and copper (2.6%) dispersed in the glass. The absorption of light by the gold particles gives the cup a red color, while the larger particles reflect the purple color. Lastly, the green color results from the absorption of light by the silver particles.