Smooth Basalt
A little-known feature on Curiosity is this piece of basaltic volcanic rock from a lava flow here on Earth. The piece has been smoothed down into a nice circle. The rover carried this bit of our planet to Mars in order to calibrate its APXS instrument against a known target. Tiny pieces of Martian dust have already accumulated on the rock -- meaning that during its calibration, APXS also got in its first Martian soil analysis, showing sulfur chlorine in these grains.