Could diamonds be formed from coal?
Craters actually can be identified by formation of high-pressure materials such as diamonds or stishovites and coesites (varieties of shocked quartz). A good example of this is the Popigai crater in Russia. 

Pressures and temperatures at subduction zones are usually too low to form big diamonds but microdiamonds (10-80 microns) were discovered within metamorphic rocks related to continental collisions. There are five well confirmed diamond-bearing terranes, the Kokchetav massif of Kazakhstan, Dabie and Quinlin in China, the Western Gneiss Region of Norway, the Erzgebirge massif of Germany, and the Kimi complex of the Greek Rhodope (according to the paper http://www.pnas.org/content/104/22/9128.long).