according to google:
link
the site says it's a member of the quartz family and is often used in jewelry. So i would think that unless you've had some sort of reaction to amethyst, there wouldn't be any allergic problems with this.
New Age properties i found include good luck, healing, and more related: it eases anxiety, that is has "projective" energy, that it strengthens your libido and "strengthens your blood and muscle tissues."
seems appropriate for a cock ring