It is a variety of chalcedony.
Color ranges from grey to smoky brown, to brown-black.
Used in thought transference, for sending ideas, emotions, and messages, and receiving the nurturing energy of the universe. Ancient tribes have used this stone to cleanse haunted locations. Also used to muster strength physically and psychologically for confrontations, making it useful for providing insight in opposing contrary many forces. Flint has also been used in psychic surgery as a tool.