Curiosity is doing a number of cool thing on Mars. One of which is discovering (and analyzing) meteorites on Mars.
This rock is an iron meteorite named "Lebanon" (the smaller one in the foreground is called "Lebanon B"). The main rock is about 2 meters wide (which is left to right). Both Spirit and Opportunity have found meteorites, analysis of Lebanon reveals it to be a similar shape and composition to the meteorites found by the other rovers.
This picture is a composite of high-resolution images taken by the Remote Micro-Imager and the Mast Camera on May 25th. Here, we can clearly see a number of cavities in Lebanon, but they have an angular shape to them. This is probably explained by either erosion from the Martian surface or that the crevasses once contained olivine crystals that have since been lost to time.
On Earth (at least), whereas iron meteorites aren't rare, they aren't as common as their stony siblings. Thus far, we have found mainly iron meteorites on Mars, which is probably because iron is more resistant to erosion. (On Earth, we have the advantage that we can find stony meteorites before they've eroded away).
You can look at a full, more high resolution image, of Lebanon here.