1. A method for classifying a composition of a sample of unknown composition, the method comprising: illuminating a portion of the sample with a first beam of penetrating radiation, the first beam characterized by a first pre-specified set of beam settings; detecting x-ray emission emanating from the sample; illuminating the portion of the sample with a second beam of penetrating radiation, the second beam characterized by a second pre-specified set of beam settings; performing a complete spectral analysis of the x-ray emission; and categorizing the composition of the sample as one of predominantly non-metal and predominantly metal based on the complete spectral analysis.