Cubical rocks lie in a spot designated “Chocolate Hills” by the JPL team. These remind me of stones quarried to build ancient structures on our planet.
Excerpt from the Mars Rover website:
Opportunity at a sweet spot on Mars
This rock has a thick, dark-colored coating that is interesting to scientists because many of the rocks in the surrounding area have the same mysterious dark stuff. The coating could be remnants of a layer that was changed by the action of water and weather or, it could be a layer of rock that melted when a meteor (less than a foot across) impacted Mars, ejecting this rock and others and creating the crater “Concepcion”. Knowing its origins will help them understand the history of Mars. Opportunity’s mission is to figure out the “ingredients” of this morsel by studying the chemicals in it.
The article is silent about the shape of the rocks.

“With malice toward none, with charity for all, with firmness in the right as God gives us to see the right, let us strive on to finish the work we are in, to bind up the nation’s wounds, to care for him who shall have borne the battle and for his widow and his orphan, to do all which may achieve and cherish a just and lasting peace among ourselves and with all nations.” — Abraham Lincoln