Jones was a very talented musician. He played sitar in "Paint it black" and the dulcimer in "Lady Jane". He also played the second guitar behind Keith Richards.
In June 1969, Brian Jones decided to quit the group. It was explained as differences over the group's musical direction. It may have been "differences" but Jones had terrible addiction problems. He was missing studio sessions and was completely unreliable.
A month later, or July 3, 1969, Jones was found dead in his swimming pool. His addictions apparently caught up with him.