Mars Rover Curiosity's mission isn't just one that represents the future of space tech; it's also about uncovering the history of Mars. Minerals can be a strong indication of what the planet looked like as it was forming. Certain minerals, for example, may indicate that lava once flowed near a certain area. The chemistry and Mineralogy X-Ray Diffraction Instrument (CheMin) will be able to find and analyze those and a whole lot more.
Curiosity will be able to drill into rocks and collect a powder, then store it internally. CheMin will shoot tiny X-rays at the rock or soil sample; when they interact with it, some are absorbed and re-emitted at different energies. By calculating those energies, CheMin will be able to determine the atoms present in the sample.
What minerals they discover might also hint at how much of a role water played in forming the planet's minerals. Certain minerals contain water, and CheMin can tell the difference between those and the waterless variety. It might even clue scientists in on if Mars could have once supported life.