how did 4th-century romans make a cup that turns RED when you light it from inside

not a repost, genuine 'how the hell' question. the Lycurgus Cup, in the British Museum, Roman, ~4th century AD. look at it in normal light from the front = jade green, kind of dull. shine a light THROUGH it = glowing blood red. same cup. it's carved as a cage-cup, full 3D scene of king Lycurgus get……