Onyx primarily refers to the parallel banded variety of the silicate mineral chalcedony. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands. The colors of its bands range from white to almost every color.  Onyx is rated at a 7 on the Mohs hardness scale.  Onyx is reffered to as a grounding stone in the metaphysical world. Onyx is thought to increase vigor, strength, stamina and self-control, as well as eliminate confusion and alleviate nightmares.