Black onyx is ultimately known to be a protective stone. It lends one the power of strong defense by absorbing negative energies and especially assists those who feel like they are being drained of personal energy. A stone of discipline, it aids one in achieving and "grinding" out their goals and stimulates good decision making.
This stone strengthens one's confidence, making the person feel more secure by stabilizing mental and physical imbalances. It will provide the structure that one craves, and to be the master of their own future. One will master the creation of their future by making wise decisions and becoming the driving force of their own lives.
Info by: https://thecrystalcouncil.com/crystals/black-onyx