It looks like a stone I've seen called mystic topaz.
"Mystic topaz is quite a new gem type, first seen around 1998. It is natural colourless (white) topaz that has been coated, giving it a unique rainbow colour effect. Therefore, it is not a gem type, but is an enhanced clear topaz. Mystic topaz may be so-called because its attractive changing colours appear deep, mysterious and unusual. Mystic topaz usually appears to display rainbow colours, with greens, blues and purples being quite prominent. Since the treatment is a coating, it is not a permanent enhancement, but its remarkable appearance makes it an interesting gem in its own right."