"Heart of Stone" was a Top 20 hit for the Rolling Stones in 1965 -- their first original song, in fact, to make the American Top 20 (although "Tell Me" had come close to doing so). At a time when the group was not writing that much original material, and at a time when much of the original material they were coming up with wasn't that good, this must have been a real shot in the arm to their self-confidence. "Heart of Stone" was a slow and soulful, dramatic ballad with the kind of vaguely discordant, droning guitars heard on many an early Rolling Stones slow number. What was impressive was how the Jagger/Richards song, though similar in some respect to American soul ballads of the period like "Pain in My Heart" (which the Stones covered around this time), was not explicitly derivative of any one blues or soul song that they were covering on their mid-'60s records. The lilt of the verses owed something to country music and the mournful harmonies heard on the latter part of the verses added to the overall feeling of melancholy moodiness. Mick Jagger's vocal had the kind of studly insouciance that was becoming typical of his lyrical stance, with the boasts about how many girls he's known (probably in the Biblical sense) and burned. The chorus dramatically brags that the woman he's singing to will never break his "Heart of Stone" -- although there's a tinge of hurt and vulnerability that hints at some fear and insecurity masked by that boast, emphasized by the ringing minor chord that immediately follows the phrase. The guitar solo is a marvel of elegant yet skeletal blues simplicity. Oddly, despite its respectable success in the U.S., "Heart of Stone" would not be issued in the Rolling Stones' native U.K. for about a year, and then only as an album track. Perhaps the group was not confident enough about the merits of an original composition to yet chance a British single release, or perhaps it was just that the American market was far more accommodating of more single releases per year than the British one was. Or perhaps they wanted to take in some money from publishing.