Unfortunately, Horton, I haven't had any time to look at this as yet. I am sure there are some who have a conclusion based upon their expertise in geology, or lengthy field work in deserts.
On Earth, I have seen much 'shiny' material, eroded to a glassy surface, or with the inherent qualities of discoloration/alteration at the immediate surface, and those compete with the ID of a 'desert varnish' as a possible biological coating.
I can tell here from the subject we needed a color camera, and multi-mega-pixel imaging to perform the proper identification by visual clues and simple observation. This rock is upon a field of well differntiated materials, which also at times seem to have a surface coating or formation 'rind'.
Hopefully a closer look will give some clues, additional to the other rover instrument reports.
I notice the 'layered' effect, or orientation, which indicates either molecular organization, or stress applied. The rock was altered at some point, as was much of Mars, and may be resurfaced after the orientation process. Possibly the surface is newer than the rock?
The shape you are showing doesn't seem to be a stress induced shape or pattern. It is well ordered, and relaxed, in overall shape.
