Diamonds burn and vaporize in the air
Although diamonds are very resistant to mechanical damage and high temperatures, given enough temperature and oxygen, they can burn.
When a diamond, which is composed of pure carbon, burns, it reacts with oxygen in the atmosphere to produce carbon dioxide (CO2).
This reaction requires high temperatures, usually above 700 degrees Celsius, to initiate the combustion process.
Thus, while diamonds are very stable, they are not invulnerable and can be burned as well as completely vaporized under certain conditions.
In conditions where oxygen is not available, such as in space, diamonds can retain their shape forever.
There, they remain unharmed despite extreme temperatures.