The formation of low-albedo slope streaks represents one of the most active surface processes presently observed on Mars. Ongoing debate about the origin of such streaks centers on whether they are the products of: 1) erosional processes related to `dry' mass-wasting exposing a dark-toned substrate beneath a thin dust mantle; or 2) `wet' processes associated with liquid seeps. Here, we employ hyperspectral CRISM images to determine the spectral properties of individual slope streaks, constrain their composition, and test possible formation mechanisms. Our results demonstrate that the slope streaks analyzed here form a spectrally distinct and compositionally unique class of active Martian surface features. Darkening through exposure of a pre-existing substrate, textural effects and/or persistent soil moisture are all unambiguously ruled out by the spectral observations, which instead point towards a transparent surface coating or enrichment in low-albedo ferric oxides as the most likely and spectrally permissible mechanisms.