Onyx is strength-giving. It provides support in difficult or confusing circumstances and during times of enormous mental or physical stress. Centering your energy and aligning it with a higher power, accessing higher guidance, is facilitated by Onyx as is a connection with the whole. This stone promotes vigor, steadfastness, and stamina.
 
Source Credit: The Crystal Bible by Judy Hall
Black Opal is a stone of magic and mystery. It stimulates and links the root and crown chakras, assisting one in connecting one's highest spiritual aspirations with one's physical life. It is a powerful magnifier of one's intention, especially if that intention carries an emotional charge.
 
Soucre Credit : The Pocket Book of Stones by Robert Simmons