Black onyx is known as the "stone of strength". It is a stone of self-mastery, giving one self control & anchoring one's flighty energy into a more stable way of life. Onyx promotes inner strength, determination, and endurance to complete difficult tasks or draining activities.
In addition to grounding one's energy, onyx is also said to dispel negative energy & promote emotional well-being. Look to this stone as a source of support during times of mental or physical stress.
Onyx is also a great stone for those of us who have an abundance of ideas, but lack the discipline to carry those ideas out. This stone fosters wise decision-making & prevents the drain of personal energy, helping one master their future.
Onyx is associated with the Root Chakra, and can help with grounding, protecting, and balancing by providing an anchor for your energy, thereby giving you focus. It's an excellent stone for meditation and dreaming, especially when used in combination with other grounding stones.