Guitarist K.K. Downing and singer Rob Halford of Judas Priest perform at London’s Hammersmith Odeon in 1983. (Pete Cronin/Redferns)

Folk singer Joan Baez has dedicated much of her career to interpreting the songs of others, but there are few memorable examples of the reverse — musicians covering songs that she wrote. The startling exception is Judas Priest’s epic shredding of “Diamonds & Rust.” The British heavy metal band put the song on its third album, “Sin After Sin,” in 1977, after Baez released it as the title track of a 1975 album. The thunderous cover version became a fan favorite at Priest concerts. I recently asked Rob Halford, the lead singer and primary lyricist of Judas Priest, who is known for riding a motorcycle onstage, to tell me the story of how this unthinkable musical match came to be. He spoke by phone from Germany, where the band was on its “Firepower” album tour.

“It’s a vivid memory, actually,” Halford said. “Here’s the thing: Judas Priest were just starting to gain some kind of traction in America ... and the label had suggested to us that we might want to consider doing a cover song to see if we could make some kind of entrance into the glorious American rock-and-roll radio scene. So, you know, Priest then, and Priest now, has always been very open-minded to any opportunity to spread the gospel of Judas Priest metal. So we said, ‘Yeah, send anything, we’re ready to hear whatever you’ve got.’ ”

“The first time we heard that song, we were in a little studio ... in South Wales. ... This 45 single came in the mail, registered delivery from the label. ... We take it out and there it is, a 45 single, Joan Baez, ‘Diamonds & Rust.’ We go in the studio and open an old record player and put the single on the top and press start and watch it drop and the needle go, and then the music starts. Now bear in mind, we all know who Joan Baez is. We’re all aware she’s a very, very famous lady, famous musician, famous activist. [But] we have no idea what was going to come through the speakers. So when we first hear Joan, simply Joan and her acoustic guitar, singing the opening lines, ‘I’ll be damned, here comes your ghost again,’ we all looked at each other. And the initial gut feeling was, ‘This can’t be right. This isn’t heavy metal! How can this be turned into a heavy metal experience?’ But then as the song moved on, I think it was like a bit of an epiphany for Judas Priest. Because at that moment we understood that a great song, as is the case with Joan Baez, ‘Diamonds & Rust,’ will take any kind of translation, any kind of changing direction, changing sound, however you want to describe it. So then for us it became a real fun, interesting experience of taking this beautiful, very fragile song, and making it into this big metal monster. Which it still is for Judas Priest. ... It’s still a beloved song. When Judas Priest fires up that track, it instantly gets the room rocking.”

Baez wrote the song about her old lover Bob Dylan, and the song contains some poetic imagery that one does not associate with metal. I asked Halford about his decision to cut a couple of those verses. He said: “Yeah, the ‘eyes bluer than robins’ eggs.’ Yeah, we had to. The metal world, as you know, is a very strong experience, so you have to kind of — I wouldn’t say take liberties, but you have to be aware of what you need to do. So we respectfully omitted that verse.” When the band plays the song live, Halford said, “I always think about Joan when I’m singing it. ... I know how she’s feeling when she’s saying those words. ... I’m thinking very much about the message of what she’s talking about, who she’s talking about, talking about Bob.”

Joan Baez performs at the “War Is Over” rally in New York’s Central Park in May 1975. (Allan Tannenbaum/Getty Images)

But Halford didn’t get to meet Baez until 1985, when both were playing Live Aid in Philadelphia. “We’re down in Nassau in the Bahamas making the ‘Turbo’ album when we get a call from the Live Aid people via Bob Geldof and everybody: Would the band come up and do Live Aid in Philadelphia with all the other performers? So we just dropped everything and caught a flight. ... I’m just finishing an interview with Martha Quinn of MTV, and Martha’s wrapping up, and she’s leaving, and then we’re all backstage. And then I see Joan walking towards me. And I’m thinking, Oh my God, she’s going to give me hell for what we did with ‘Diamonds & Rust.’ And I’ve never met Joan Baez before in my life. And she comes waving, ‘Hi, Rob.’ ‘Hey, Joan, it’s lovely to see you.’ And we gave each other a hug. And she said, ‘I knew you were playing today, and I just wanted to come over to let you know that my son said to me, “If you see anybody from Judas Priest, will you please tell them that I prefer your version of ‘Diamonds & Rust’ to my mom’s version of ‘Diamonds & Rust’?” ’ And I’m thinking, Oh my God, this giant in the music industry is so self-effacing, so self-deprecating. Just such a beautiful thing to say. Of all things, to come and say that to us. That just speaks volumes about the humanity of Joan Baez. That was really a nice little tie-up of that whole experience into Joan Baez’s world of music.”