Mars Anomalies Sol 721 - Building-Craft or trick of light?

Another interesting anomaly has been found on Mars by the Mars Curiosity rover on Sol 721. Could this be a building, a craft or is just a nice rock?

NASA Link: http://mars.jpl.nasa.gov/msl-raw-images/msss/00721/mcam/0721MR0030860060402822E01_DXXX.jpg