Oddly Textured Rock ID

[tekknoir]

Hi all,

I'm trying to figure out this odd rock I found today. About 3/4 inch long, textured on one side, broken on another and smooth aside from some dimpling on the rest. Found in Doña Ana county, New Mexico. It's a very fossil rich area with an abundance of petrified wood, but I'm not sure this falls into either of those categories. Any ideas?

Thanks!

[jasong]

I don't recognize fossils generally unless they are obvious or something I've seen before, so if it's that then I wouldn't know. The fact there appears to be a vein in it would make me guess rock, not fossil though initially. 

In general it appears to be a cobble exhibiting differential weathering. Can't tell from the pic if it's like hematite or something like chert though. But if I took a total guess at where to start looking/testing, that's where I'd begin. It could be a wind or water polished cobble, or it could be something like a nodule or concretion. A good start would be a scratch (hardness) test and a streak test. That'll ballpark you in terms of composition. But if it's something like a fossil then I dunno. 

Differential weathering is fancy term for the process that occurs during natural erosion where two dissimilar minerals in one rock weather at different rates due to different hardnesses, chemical susceptibility, etc. It can form dimples, cracks, and textures like that. Normally spoken in terms of large formations, but applies just as much to small cobbles.

Maybe someone else recognizes it in particular. Sometimes various things like that can have specific names from specific locations too, beyond just ID'ing their mineral makeup (desert roses/barite&gypsum, etc)

[Geologyhound]

The textured surface makes me think bone fragment.  Once fossilized, fossils (like any rock) can be fractured and the fracture can then be filled and “healed” over time.  I am guessing the dark shiny surface is either a coat of desert varnish, or wind/sand polish.  I have found fossil vertebrae with similar black, shiny surfacing along the Oregon coast.