Diamonds very slowly degrade into graphite … but the sun, the solar system, and many of the black holes will have died long before then.
When thinking about diamonds we can think of the Shawshank Redemption line that, “Geology is the study of pressure and time. That’s all it takes really… pressure… and time”. Diamonds mined from the Earth are carbon atoms that have been compressed over long periods of time (in the billions of years) under enormous amounts of pressure.
The sands of time
The underground pressure that forms a diamond also holds it together. Once removed from the ground the carbon atoms very, very, slowly rearrange into graphite. Graphite is a more stable arrangement of carbon atoms than a diamond, but “more stable” is extremely relative. Under normal conditions a diamond sitting in your house would take an estimated 10 to the power of 80 years (1080 years) to become graphite, which is:
One with 80 zeros after it, years. To put that in perspective, the universe is only around 14 billion years old, or 14,000,000,000 years old. A wildly shorter period of time.
The slow death of the universe
So what will life be like when diamonds begin to lose their luster? Nothing lasts forever, including our sun and the universe as we know it. Our sun is scheduled to become a red giant star, expanding in size to engulf Mercury, Venus, and probably Earth, in about 5 billion years. But before that happens the increasing brightness of the sun will kill off all life on Earth in about 1 billion years.
If humanity takes its diamonds aboard a spaceship (with standard Earth like conditions), and sails the universe through space & time, it will theoretically take until the Black Hole Era for the diamonds to become graphite. The Black Hole Era will be from 1043 to approximately 10100 years from now. Leading up to the Black Hole Era the stars will have burned out and the planets (and your diamonds) will have decayed because their protons fell apart. In this time of darkness the black holes of the universe will decay and evaporate into nothingness … and then, finally, your (theoretically still existent) diamonds will have become graphite.
Added info: in 1947 De Beers launched the marketing campaign that “A Diamond Is Forever” (which they used to create our modern idea of the engagement ring). Also learn more about the carat measurement of diamonds (and the other karats, carets, & carrots).