> Maybe it was the same type of crystal with different impurities?

Most likely. You can get things like gradations between amethyst (a purple type of quartz) and citrine (an orange type of quartz) in a single crystal because it's all quartz with different things substituting into the lattice. Some minerals can have really complicated intergrowths and gradations of versions themselves, e.g., tourmaline does all sorts of weird stuff, but importantly all have effectively the same lattice structure.