When diamond is heated at more than 1500 Degree Celsius in the absence of air, then it turns to what?

When diamond is heated at more than 1500 degrees Celsius in the absence of air (in an inert atmosphere such as argon), it does not melt directly into a liquid. Instead, diamond undergoes a transformation into graphite. This transformation is a structural change where the carbon atoms rearrange from a tetrahedral crystal structure (diamond) to a planar hexagonal crystal structure (graphite).

Graphite is a soft, black, lustrous form of carbon, and it is the stable form of carbon at standard temperature and pressure after diamond.