Does anyone know what this stone is called?

Awhile ago I was shown a necklace. It was in a single diamond pendant style. I am almost certain it was not a real diamond .It looked multi-colored or rainbow as you twisted and turned it. It had colors like purple, green, orange, and yellow. Does anyone happen to know what I am talking about? I know it's pretty vague on details, but it was over a year ago that I saw it and I can't seem to remember exactly what it was called and I can't locate any pictures of it. I just keep getting pictures of necklaces that are literally rainbow colored.