I had no idea what a "rainbow cup" was, so:
"The name rainbow cup coins come from the legend that they are drops of gold that fall to earth at the end of a rainbow," Ziegaus told Live Science in an email. "Another legend about these Celtic coins tells us that these coins can only be found by Sunday children," or a child of fortune.
"In fact, the finder was born on a Sunday and is indeed a Sunday child, a lucky child!" he said.