Obsidian is a volcanic glass that is sometimes worn in necklaces as a protective stone. It is said to have many benefits, including: 
Protection
Obsidian is thought to block, absorb, and transform negative energy, acting as a shield against psychic attacks. It is also believed to protect wealth and health. 
Emotional healing
Obsidian is said to release emotional blockages, stress, and tension, and can help clear ancient traumas. It can also help Scorpios navigate intense emotions and Sagittarians stay grounded while exploring new experiences. 
Grounding
Obsidian can help people connect with their inner power, intuition, and inner strength. It can also help people access truths they may be keeping from themselves. 
Personal growth
Obsidian is said to encourage personal growth.