The note of caution provided by Hargraves et al. concerning the use of JSC Mars-1 as a simulant, in the strict sense, to Mars soil is correct. JSC Mars-1 simulates bright region spectra well and approximates Mars soil elemental composition, particle size distribution, magnetic properties, moisture content, and physical properties with lesser degrees of precision. We measured these properties and published them, along with comparative data for actual Mars soil. Researchers should use JSC Mars-1 or any other “reference material” with caution, bearing firmly in mind the differences and similarities.