Ok, I want to start off by stating I have very little formal training in geology, At 15 I spent 6 weeks in the 4 corners area learning some field geology, and had 2 college introductory courses in it... This means I only have a small grasp of geology. But there are a few things about this rock that I would like to speculate on...

First off- it is laying on the surface- this indicates it is crater ejecta from a meteorite impact, given its size it probably wasnt thrown very far from the impact- this means it could be from the original 'lakebed' of Gusev

Secondly- the left side of the rock shows a long crack the seems suggestive of a a fracture between layers... In other words it looks like if you took a geologists hammer to that side you could peel off a slab by extending the crack along the plane of fracture...

This would imply that there are two somewhat different "layers" in this rock and the fracture has developed between them... This _suggests_ that the rock was formed by some form of layered deposition....

Now to go WAY out on a limb-

In my LIMITED experience, non aqueous deposited sandstones-(Ie fossilized sand dunes) do not exhibit fracture planes, while those deposited in aqueous ones do...

So from just the picture, is there ANY justification for getting hopes up that this may be a sedimentary rock from an old lakebed?