Exemplary embodiments of this invention encompass a method for purifying crude rebaudioside A. In particular, this invention relates to a method for purifying crude rebaudioside A compositions comprising purities from approximately 40 % to approximately 95 % rebaudioside A to obtain a substantially pure rebaudioside A product with a single crystallization step. Resulting polymorph and amorphous forms of rebaudioside A and methods for preparing polymorph and amorphous forms of rebaudioside A from crude rebaudioside A compositions and substantially pure rebaudioside A compositions also are disclosed.