According to MSL scientists based at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., the ball isn't as big as it looks - it's approximately one centimeter wide. Their explanation is that it is most likely something known as a "concretion." Other examples of concretions have been found on the Martian surface before - take, for example, the tiny haematite concretions, or "blueberries", observed by Mars rover Opportunity in 2004 - and they were created during sedimentary rock formation when Mars was abundant in liquid water many millions of years ago.
By now we all know that Mars used to be a lot wetter than it is now. Curiosity quickly worked out that it was exploring an ancient lakebed shortly after it landed inside Gale Crater in August 2012. That ancient lakebed is characterized by obvious layering of sedimentary rock. On Earth, sedimentary rock is formed through the interaction of liquid water transporting and depositing material - the same process also occurred on Mars.
Within the newly forming sedimentary rock, pores are inevitably created and minerals seep into those pores, gradually building up an erosion-resistant mass. Over time, as the soft sedimentary rock is eroded away, the concretion remains behind. And this little sphere is one such example - the ball has either emerged from the underlying sedimentary rock that has eroded away or, perhaps, it rolled from somewhere else over time.