Obsidian is a powerful cleansing stone, and should be wary when using it to help with healing rituals. 

In general it was help to enhance the spiritual properties of herbs if placed within the jar, help to shield against negativity and help to absorb negative energies from the environment. 

Obsidian is sometimes also used as a handle for a magickal knife, as it can help the most experienced wielder control the energies so they can focus and channel it much better.

Ritualistic uses - 

Scrying

Healing

Dispelling of Negative Energy

Protection

Aiding in Spiritual Work

Element - Earth and Fire

Zodiac - Scorpio and Sagittarius 

Chakra - Base