1. A method for classifying a sample by material type, the method comprising: obtaining measurements of reflections from a sample in at least two spectral bands at respective illumination angles and respective observation angles, wherein the measurements of reflections are obtained from one or more cameras, wherein the at least two spectral bands include a first spectral band and a second spectral band, wherein one or more projectors illuminate the sample in the first spectral band at a first illumination angle, wherein the one or more projectors illuminate the sample in the second spectral band at a second illumination angle, and wherein the first illumination angle is different from the second illumination angle; identifying, with one or more computing devices, specular measurements in the at least two spectral bands; and calculating, with the one or more computing devices, a feature value for the sample based on a ratio value of a specular measurement in a first spectral band of the at least two spectral bands to a specular measurement in a second spectral band of the at least two spectral bands.