re: Harder?
I think our poor confused friend isn't referring to surface hardness and scratching as in Moh's scale, but more to physical breakdown and large-scale erosion by scraping and loading. This measures a different sort of strength and also determines if the material is fractured or not.
Fear not, young Mr. Clark, there will be plenty of samples of white crusts as the mission proceeds. Once we get out of this crater, things are going to be very boring out there and they'll be desperate to find something different to do to maintain public interest.