Today is Keith Richards's 75th birthday, which on its own would be a momentous occasion for the Rolling Stones guitarist who has cheated death so many times, people often can't remember if he's alive or not. In fact, there's even a phenomenon called the Keith Richards Effect, wherein Google Search traffic for "is Keith Richards dead?" spikes every time another famous rocker, such as David Bowie or Tom Petty, dies.

But December 18 is a special occasion for Richards for a couple of other reasons. Today is also his 35th anniversary—Keith met American model Patti Hansen in New York in the late '70s, and they married in 1983, on his 40th birthday. (Incredibly, Hansen told Vogue, they connected at the Roxy roller rink during his 1979 birthday party, and "have been together ever since.")

This year also marks another important anniversary for Richards, though. It's the 40th anniversary of a birthday gift that is so closely associated with Richards and his aesthetic that it’s now hard to picture him without it. No, not his eyeliner or headbands or custom Fender Telecaster. His right-hand skull ring.

In 1978, Richards was planning a blowout 35th birthday party in New York. Among those invited was David Courts, a longtime friend who was a craftsman and jeweler in London.

"At the time there was a big connection between the art scene and the music scene," Courts told Richards biographer Victor Bockris in 1992. "I think it had a lot to do with a lot of musicians coming out of art school, and so it was quite easy to jell. I started making jewelry for Keith through Anita [Pallenberg, Keith's longtime girlfriend]."

Courts said that his work for Richards started in the 1960s with a skull pin, which he'd adorned with a bishop's miter covered in sapphires, rubies, and diamonds. Pallenberg immediately wanted it custom-engraved for Richards, and after that, Richards was a regular customer of Courts's often-macabre works.

By the late '70s, Courts and his business partner, Bill Hackett, had been working on making realistic miniature skeletons. Using a real skull for reference, they had carved out a detailed mold, set it in silver, and created the large, heavy ring. Immediately, they knew that Richards was meant to have it. "From the moment he put it on his finger," their site declares, "the magic began."

For decades, Courts and Hackett never made replicas. Keith's ring was featured on album covers, magazine spreads, seen live at every concert—essentially the best possible product placement imaginable—but they never appeased fans by making the ring commercially available. That is, until 2009 (and with Keith's blessing, their site points out). Called the "Death's Head Skull Ring," it's sculpted from the same skull and features the same cranial and nasal detailing as Keith's. The solid 925 sterling silver ring can be custom-ordered to size for roughly $440.

As for Richards, his famous memento mori has resonated with him as much more than just a fancy talisman. "[My skull ring] is to remind me that we're all the same under the skin," Richards told Rolling Stone in 1988. "The skull—it has nothing to do with bravado and surface bullshit. To me, the main thing about living on this planet is to know who the hell you are and to be real about it. That's the reason I'm still alive."