Mining natural diamonds damages the environment unless you restore the original landscape after the extraction is complete. In some countries the mining results in war and conflict, because of its high value.
To produce synthetic diamonds you need extremely high pressures and temperatures, therefore colossal power generated probably by fossil fuels (bad for the environment, more pollution).
While I am telling you all that, a friend of mine and myself are investigating a site near here that probably has diamonds in shallow depth under a forest. If we want to mine them, we have to do an awful lot of environmental studies to get a mining permission. After eventual mining we have to restore the landscape as it was originally. A big operation!