Interesting comments so far. Thanks for the replies. To follow up...
Cutting the piece (with my portaband using a 14 TPI band) did not ruin the blade. It was slow going, but no real problem.
Likewise, sanding it wasn't too difficult either.
I haven't had a chance to burn a piece to see if it gives off the distinctive mesquite smell. I might do so this weekend, if I remember. Somehow, I doubt it will.
Not sure if there are any other tests that might confirm or refute any of the ideas, but if you think of any I'll be happy to try some out, assuming I have the tools, resources or talent needed.
As an aside, the first picture I posted was an image of the way I received it from the "pretty" side (no sanding involved). If I'd flipped the piece over and shown you the other side it looks similar, but kind of "washed out" (as if it sat in a small pool of water or something). When I cut and sanded the two pieces I made, that "washed out" look proved very superficial. Not sure if that gives anyone an additional clue.
- Greg