The lyric turns on the song’s central image — hearts turned to stone, hardened by heartbreak so that they can no longer feel or love. That stark figure of emotional numbness, set against Jimmy Martin’s hard-driving, confident sound, gave the song a memorable bite.

“Rock Hearts” settled in as a bluegrass standard. It was revived for a later generation by the Bluegrass Album Band, the all-star group whose recordings of traditional material reintroduced songs like this.