All natural diamonds and the vast majority of synthetic diamonds are created in HPHT conditions, that is, high pressure, high temperature environments.  It is in these conditions that carbon is formed into a latticework of isotropic crystals that are usually transparent, resistant to impurities and are superior in hardness and thermal conductivity.

Synthetic, or manufactured, diamonds are, however, more difficult to make as gem quality stones.  Therefore it is more typical to find manufactured diamonds as having similar qualities to natural diamonds, but without the visually and aesthetically pleasing characteristics of natural diamonds.  Synthetic diamonds are more often used in an industrial capacity, as part of cutting and polishing tools as opposed to being used as gems for public sale.  Also, with manufactured diamonds, the stones may be of either superior or inferior nature in hardness, crystallinity and inclusions, depending on the method used to manufacture them and its quality control.