The Roman glass chalice shown here is from the 4th century C.E. It's called the Lycurgus Cup, and it can be found in the British Museum. That's King Lycurgus of Thrace himself on the exterior of the vessel. When the cup is lit from the front it appears green in colour, however, when lit from the rear, it turns a deep shade of red.
It was not until 1990 that British scientists were able to understand why this occurs. Close examination of the cup's glass under a microscope revealed that it contains extremely small particles of gold and silver – pieces smaller than one-thousandth the size of a grain of table salt. These nanoparticles absorb and refract light in strange ways, an effect modern science calls “dichroism”. The Romans were the first to experiment with this process, making the Lycurgus Cup an early example of dichroic glass. You can see this same principle at work with certain stained glass windows found in European churches and cathedrals.