Under high pressure and intense heat, graphite, the most stable form of pure carbon, changes into the substance commonly referred to as diamond and remaining this way whether or not the heat and pressure are removed.
Under high pressure and intense heat, graphite, the most stable form of pure carbon, changes into the substance commonly referred to as diamond and remaining this way whether or not the heat and pressure are removed.
if you use "although", you are basically declaring that "the heat and pressure are removed". in other words, "X, although Y" expresses the idea that X and Y are both facts (although they contrast with each other in some way). this is not the intended meaning; "heat and pressure are removed" is a hypothetical that may or may not happen, and so should be introduced with "when" or "if" rather than "although".