Black Onyx is a variety of Chalcedony, which is member of the quartz family. It's associated with the Root (1st), Solar Plexus (3rd) and the (Third Eye 6th) and its element is Earth. Black Onyx is a stone of strength and self mastery and is said bring about an increase in one's personal power. Onyx is a great stone to have in your arsenal when dealing with conflict when one must remain calm.
*Gemstone information gathered from "The Book of Stones" by Robert Simmons and Naisha Ahsian. Pick this book up for an in-depth description of almost every stone on Earth!