When metallic particles have dimensions on the nanoscale, they display iridescent colours. The first known example of this phenomenon is the Lycurgus cup, a 4th century chalice made of glass impregnated with particles of silver and gold, ground down until they were as small as 50 nanometres in diameter, less than one-thousandth the size of a grain of table salt. This produced the optical phenomenon, known as dichroism, which occurs when the colour of the cup changes from green to red according to the position of the light source.