[Mahmoudfel]

  Is it true that we can produce diamond from carbon ? and how?

[coldest]

of course we can.

carbon has many phase, such as diamond, graphite, carbon nanotube, C60 fullerene, carbon onions, carbon spheres etc. they could transform from one to another at some temperature or pressure. In our school, one student synthesis diamond from carbon nanotube at a low temperture through SPS method( SparkPlasma Sintering), you could find some ideas in Phase Diagram for Carbon

http://www.me.berkeley.edu/diamond/submissions/diam_intro/cphased.htm

[xiankai]

basically at like very high temperatures and pressures, u are reforming the crystal structure into that of diamond, since the particle movements will be highly erratic and thus create an irregular structure of carbon when cooled, should be something similar to diamond.