What happens to diamonds if they are exposed to high temperature and decreased pressure for a long time during emplacement?

When diamonds are exposed to high temperatures and decreased pressure over an extended period, they can indeed convert to graphite. This transformation occurs because graphite is the thermodynamically stable form of carbon at the high temperatures and lower pressures that may be experienced during certain geological processes.

Under such conditions, the crystal structure of diamond, which is characterized by a strong and rigid lattice, becomes less stable. Consequently, the carbon atoms rearrange themselves into the more stable structure of graphite, which has a layered and more flexible structure.

This process highlights the influence of environmental conditions on the stability of different forms of carbon, illustrating that diamonds are not permanent and can be altered by changes in temperature and pressure. The other potential outcomes, like remaining unchanged, turning into new diamonds, or dissolving in magma, don't accurately describe the effects of such environmental changes over prolonged periods.