Is this due to an unusually high albedo? It might be just a flat face with high specular relflectance, but there is no other like it in any other field of view, so it seems statistically unlikely. Perhaps it is a boundary between radically different sediment layers, deposited as ejecta from some meterorite hit. An evaporite?
The shadowed side is consistent with the lightness extending through the body of the rock. And there appears to be a darker band between the top and bottom portions.
These exposures at different times of day (note the shadows) show that there is not a lot of specular reflection.
From near center of Left (MAST_LEFT) on Sol 17 (2012-08-23 17:15:23 UTC) :