HPHT stands for high pressure, high temperature. This is a treatment conducted on certain types of diamond that can permanently alter the color. Sometimes there are characteristics that an educated gemologist can detect; many times those characteristics are only found by a professional grading laboratory that possesses advanced technological instruments that can measure or locate characteristics that are not visible by the eye.
When purchasing diamonds, it is very important to consult with a knowledgeable jeweler that will honestly and ethically reveal all aspects of the diamond's characteristics, including treatments. Ignorance is no excuse. Visit an American Gem Society professional.