The pink-purple coloration is a result of the absorption by The larger particles although the eco-friendly coloration is attributed to the light scattering by colloidal dispersions of Ag nanoparticles that has a size > 40 nm. The Lycurgus cup is acknowledged as on the list of oldest artificial nanomaterials [one]. https://www.directivepublications.org/journal-of-applied-nanoscience/