There are many types of Obsidian. Black, Snowflake, Rainbow and Peacock just to name a few. It is a glassy type of volcanic rock high in silica and has been used in making arrowheads and other tools for thousands of years.
Black Obsidian is used for Psychic protection, grounding and cleansing of negativity. It's said to be the "psychic vacuum cleaner," cleaning the auric field and can assist in breaking negative emotional patterns as well as clearing reproductive issues.
Snowflake Obsidian is black with light patches where once polished, the light patches resemble snowflakes. It is the stone of perseverance, insight and attunement to spiritual guidance. Used for past-life recall and spirit communication, it increases one's psychic sensitivity. Snowflake can assist one in becoming more receptive to healing energy and a healing attitude.
*Gemstone information gathered from "The Book of Stones" by Robert Simmons and Naisha Ahsian. Pick this book up for an in-depth description of almost every stone on Earth!