What kind of rock is this?

I found this chunk in northern Montana back in about 2009. It appears quartz-like, is gray-green in color, measures about 2"x3". Blurry photos follow:

Photo 1
Photo 2

Still blurry, but wet.

Don’t rule out the possibility of Coprolite

Thanks, but I’m looking for serious answers.

Is there a reason why non-blurry pictures can’t be posted?

Way too blurry. You’re getting good clear images of the bench. Try holding down the shutter button half way to lock the focus, then physically move the camera away from the specimen by about the same height as the specimen.

The far corner of photo 2 that is almost in focus. From that my best guess would be quartzite.

But we really need in focus photos so we can determine grain size and structure.

I’ll give it another shot. Thought I had it on macro.

As a shot at a serious answer, compare some of the images here.

Okay, let’s try these (wetted to bring out the texture/color):




It really looks nothing like those, although you likely couldn’t tell by the original photos.

It looks indeed a lot like quartz, maybe with some inclusions of mica. Can you estimate the hardness? Does it scratch glass?

My bet is that it is smoky quartz - which is a somewhat commonly found mineral in Montana (at least according to a number of quick googlings of ‘Montana quartz’, fwiw). As a kid, I collected all types of quartz’s, and that looks quite like one of my favorites (the smoky variety, that is).

ETA - that above link: if you click on the ‘rough’ section for pics at top of page, its much easier to see the comparability

I don’t have any glass around that I want to try it on, unfortunately. I have a couple of smaller pieces that I’m pretty sure are quartz; one is a nice yellow color with a bit of red on one end; the other is a very pretty green. None of the three is a color that I’ve associated with quartz in the past, but then I’m not a rockhound.

I think you may have nailed it. Some of those look very much like it. I found mine up along the Dinosaur Trail in the town of Malta.

Can we get a dry non-blurry picture?

I don’t think it’s smokey quartz, since it looks like your rock is bits of white/translucent quartz on a darker rock, not dark-colored quartz. I think more likely it’s a part of a quartz-rich intrusion into another rock type.

My somewhat-informed guess based on the local geology is that the non-quartz stuff is probably some sort of limestone or dolemite, although that part of Montana also has tons of glacial erratics so it could be some exotic thing brought down from Canada with the ice sheets.

I’m gonna say feldspar. First of all, most of the earth’s crust is feldspar so it’s an easy guess. But on top of that it’s not uniform, the black stuff appears to be inclusions in the quartz-like material. That’s what many feldspars will look like after erosion. Or just plain quartz with some black inclusions.

I’ll try to get a better-lighted dry shot this morning sometime, but thanks for the help so far.

Would feldspar have that rough glassy fracture pattern? At least, I assume it was fractured.

Okay, last shot at getting good shots. It’s remarkably difficult to photograph. All the surfaces have a glassy shine to them (some look bubbled, some rippled, another is more faceted), which doesn’t come out well in photos. I suppose it could be some sort of other mineral that was somehow encapsulated in quartz. Bright light only seems to penetrate through one area.

Anyway, three of these were taken in natural light, the last one under a CFL, which probably mucks up the colors.





It can look like that. The classic images of feldspar will show crystalline shapes that are apparent on the surface as it’s found in the ground. But after weathering it can take that kind of appearance. But so can a lot of rocks. Rock hounds would break open a piece like that to get a better look at the insides that haven’t been exposed to the weather. It could be some simple igneous formation of quartz and other minerals. Checking it under a black light or testing it with some acid might reveal more clues. It’s very difficult to positively identify any rock with just a photo.