It's gotta be rainbow moonstone (which actually looks like clear quartz with a blue sheen) - very similar to the blue sheen seen on labradorite. Just beautiful.
rose quartz, amethyst ,black onyx, white moonstone- these are my favourites. Purple has always been my favourite colour, but i tend to buy rose quartz jewellery.
Amethyst and possibly Black Onyx.
I have one Onyx ring and a bunch of Amethyst at home. (Including the necklace my mom had made for me for my birthday *the center piece of it is a amethyst heart*)