Need Help Identifying Rock/mineral

[matt0550]

I'll keep this short as this find could be of no interest at all to many - it is very interesting to me.

This two pieces of rock were found resting next to each other and immediately caught my eye due to the gold/yellow surface that was facing up as I walked by and noticed it. After picking them up it became apparent that they were two thin pieces of the same rock and that they fit back together perfectly with the two gold/yellow sides facing one another. I've never seen or found anything like it and I'm just trying to identify them.

Any help is greatly appreciated, I have a few books and have gone through carefully finding no matches to what I have here.

[Gold Seeker]

I'm guessing mudstone/shale, but the "yellow" are plant fossils.

The plant fell onto a prior layer of mud and then another layer of mud was deposited over the plant, then over 1000's and 1000's of years the mud turned into stone.

[matt0550]

Thanks Gold Seeker, im glad that is your assessment - it was my initial suspicion but I know nothing on this subject aside from what I learned in a single 200 level geology class 30 years ago. I pickup (where legal) any interesting or odd rock/stone that I cone across and you can imagine that I have alot of them lol. Thanks for your reply!

[jasong]

Another option is some sort of amphibole. Glaucophane comes to mind, but there are many others. They can form in layers in schists. The habit of those imprints looks very much like an amphibole to me and not really like a fossil.

Amphiboles are metamorphic though, so that sample would either need to be a schist or maybe slate type rock. Which it actually appears likely to be. Or potentially a concretion around a piece of metamorphic rock like below. 

Here is a random internet photo of some glaucophane (black mineral), this isn't in schst though, but very often is. 

[Geologyhound]

I am curious where and when you picked up this rock. The where and when may help identify the type of rock and the process which may be responsible.  Obviously a picture is not the best substitute for a rock in hand with a 10 power hand lens.  That being said, my initial guesses for type of rock would be shale, basalt or amphibolite.  The fact the rock split so nicely makes me inclined to think it is a sedimentary rock like shale which would have planar partings.  It is possible for fracture surfaces in other rocks to imitate shale partings at least on a small scale.

My guess is the orange material is actually a hydrated iron oxide like limonite and goethite.  A limonite/goethite crust would be relatively soft and could probably be rubbed or scraped off.  These minerals would be deposited in the fracture or parting when water and iron minerals, or iron sulfide minerals, are present.  Basically, when water is under pressure in the pore space of a rock it can hold more minerals and solution. When such water enters a fracture, the pressure drop means water can longer hold as much minerals in solution, and minerals are deposited in the fracture or parting.

As I’m sure you are aware, when water freezes it takes up more space.  As such “Frost – jacking“ is a major component in mechanical weathering of rocks.  Pre-existing features like parting planes in sedimentary rocks or fractures are ideal locations for water accumulation.  When this water freezes, it jacks the rock apart.  Take a close look at your rock and see if the “crystal shadows” are actually just absence of the orange material.  If so, I have to wonder if what you’re looking at is actually more recent ice crystal formation pushing or washing off some of the limonite in the fracture or parting as the rock was finally “jacked“ apart by the ice crystals.
 

Again, without the rock actually in hand, I am guessing.