So the Spirit rover photographed a cool rock, which NASA has dubbed “Sushi” (its neighbor is called “sashimi”):
What is this thing? Geode? Weird, extremely rectilinear volcanic bubble? Trick of the light?
Ummm… it’s a rock.
The surface texture looks like wind/dust erosion - almost like sand dunes where slight irregularities are self-amplifying.
The difference in colours looks like adhesion of light-coloured dust to a dark-coloured surface, exposed on the edge facing the prevailing wind - the pattern of surface colour on the smaller surrounding pieces is also significant.
IANAGeologist though, so prepare to see my face turn red.
Oops; one more detail I only just noticed - see the slightly darkened ring of surface soil forming a ‘halo’ around the rock, starting at (what I’m now fairly sure is) the windward side.
(prevailing wind emanating from the upper right corner of the frame, I’d say).
Aaaaaand I’ve been describing the wrong rock - you mean the one that is at the far right of the frame (invisible until I opened the window to full screen - I have image resizing switched off).
I have no idea what that is - best (but worthless) guess is that it is a void left by a bubble of gas in cooling volcanic rock - the shape is odd though, but if the bubble was elongated and the rock continued to deform as it cooled, it could easily end up with a non-circular cross-section. The edges of that rock look a little ‘bubbly’ too.
You a clairvoyant?
Looks like a fragment of a meteor hit…see all the pock marks…Hmmm. I do know one thing, you can bet spirit took a lighted shot of the inside of that thing. 
Can’t wait for them to discover something that looks “unnatural”
Looks to me like the dropping of an Elder God. When Cthulhu’s bunghole slams shut, it really slams shut.
Looks like a typical ventifact to me.
(IAA Geologist, but not a geomorphologist.)
Looks like a nice piece of yellowtail with some mackerel on the side.
Nope, I just do stupid things with more or less predictable frequency.
Given that holes are also visible in the cross-section of the smaller chunk near the bottom of the photo… maybe vesicular basalt that’s been modified by wind erosion?
See, this is why a rover, as cool as it is, will never be as good as a human. A geologist on the scene could have figured it out with a simple whack of with his/her hammer. 
Thank you Mr. Wizard(s), but as anyone can plainly see it’s a hunk of that dehydrated chicken found in Cup-O-Noodles.
Jeez, you guys always go right for the science!
= a ventifact. It is most likely a clast derived from basalt, but I’d wager that the basalt was not vesicular.
Agreed 100%!
Interesting slot rock interpretation, and a few different pics
[SeeThuArt]It’s the massive tiny garage where the giant aliens park their miniature Space Shuttles.[/SeeThruArt]
Where are the space raccoons?