Jim Darvill has written a book on the subject about the relationship between Stonehenge and Carn Meini, in an article in current Archaeology he says;...

"Then there is the stone itself: called bluestone because of its greenish blue colour when freshly quarried or when wet, dolorite is a very hard rock of volcanic origin with a regular pattern of white feldspar inclusions, like large lumps of sugar. ‘Are these’, Tim asks, ‘a rockbound equivalent of the stars of the night sky, a Milky Way trapped in stone? Do they symbolise some myth, such as the raindrops of the great storm that created the world, or the crystallised tears of the gods? Might the stone itself be held in high regard for its perceived magical properties?’.

I took this photo of a stone at Carn Meini, as you can see the quartz/feldspar? is fairly striking as he says 'lumps of sugar', though to be quite honest I don't think they were viewed as stars......

http://www.themodernantiquarian.com/post/64832/images/carn_meini.html