Just a lump of purplish quartzite. The rainbow colours are probably due to various degrees of impregnation with iron-richer chemicals (I wouldn't like to say quite which minerals might have taken-up the iron).
The quartzite is probably sedimentary rather than metamorphic, so one might call it a quartz arenite to be more correct.
Probably just a lump of quarzite; the indentations could be where mud clasts or other stones have weathered-out.
Septarian nodule; niceone; scroll down this page to find out more