Sci-fi movie from mid 90s, possibly early 90s. Involved two teenagers (a boy and a girl) going on an adventure with this old doctor with grey hair in search of some kind of purple crystals… maybe they were power crystals? Anyway, there was a skinny african american guy that was the villain and he was also looking for the power crystals. I believe there was two of these movies because in the second one, the old doctor with grey hair becomes the bad guy and the two teenagers team up with the villain from the first movie.