The Romans appreciated much more the jewels with variable colors than those with fixed colors.

Those among opals and pearls that exhibited a very strong play of colors, due to diffraction, were valued far above the gems with fixed color, like emeralds and rubies.

Thus it is likely that this kind of glass, with different colors in transmitted and reflected light, was very expensive, so it is not surprising that in TFA a letter of one of the Roman emperors is cited, where he mentions receiving such cups as a gift, because they certainly would have been considered a gift worthy of an emperor.