From their first studio recordings in 1962 until the release of "Out Of Our Heads" in September 1965, the Rolling Stones recorded some 85 tracks, of which 61 got an official release on single, EP or (compilation) album. Although he was forced to step back from the band's basic line-up in 1963, Ian Stewart played piano, and sometimes organ or marimbas, on 46 off all recorded tracks. A nice illustration of the fact that Stu, although he wasn't visible to the audience, still played an important role in the Stones' musical output.
But, as mentioned before, Stu wasn't the only one to play piano and keyboards with the Stones in those days. Andrew Oldham (read full article here): Jack Nitzsche ended up playing on the whole Stones RCA run - all their records from that time. After Sonny Bono introduced me to him, he just appeared at the sessions. I didn't ask him what he was doing there, in case he asked me for money.
There are three keyboard players on those mid-60s Stones RCA sessions. if it's a blues figure, it's Ian Stewart playing piano (Ian Stewart was the Ur-Stone who was not to become part of the group). On a few occasions when it's slightly strange it might be Brian Jones, but the rest - all the piano, organ, harpsichord playing - plus the denseness, the body, the glue - is Jack Nitzsche.
I wonder how these two great players, 'insider' Ian Stewart and 'outsider' Jack Nitzsche, got along with each other in those days. I don't know, anybody?