Tourmaline cleanses, purifies, and transforms dense energy into a lighter vibration. It grounds spiritual energy, clears and balances all the chakras*, and forms a protective shield around the body.
Hall, Judy. The Crystal Bible (p. 297).
Onyx is a stone of inner strength. It enhances one’s endurance and persistence, enabling one to carry even difficult and dreary tasks through to completion. On a mental level, it increases one’s ability to maintain focus, allowing one to learn challenging new material and use well all that one has mastered. It energetically boosts the retention of memory and encourages attention to detail.
Simmons, Robert; Ahsian, Naisha. The Book of Stones, Revised Edition