A mineral deposit? Crystal formation? Abandoned alien ray gun? Universe Today writes:
The Curiosity Mars rover has imaged a small metallic-looking protuberance which projects a shadow on the rock below. The image was taken with the right Mastcam on Curiosity on Sol 173 — January 30, 2013 here on Earth — and was pointed out to us by Elisabetta Bonora, an image editing enthusiast from Italy.
The protuberance seems different than the rock on which it sits – it could be composed of material more resistant to erosion than the rest and similar material could be within the rock, or it could be something that is “grown” on the rock. It looks fairly smooth, and is not covered by dust as is the case for metal surfaces that tend to clean easily.