As hand specimens, fine-grained rocks like this are problematic, especially from photos.
We need to look for various structures to give us clues: scattered larger well-defined crystals could indicate an igneous or metamorphic nature; depositional structures would suggest sedimentary rock. In this case, unfortunately I can't see anything useful.
So I am guessing, but yes, it could be a basalt (some are greenish, though usually darker than yours). It could be green because of a high percentage of, for instance, olivine. But it could also be a metabasalt - metamorphism having changed some of the primary minerals to green metamorphic ones such as chlorite or epidote.
However, location can constrain the range of possibilities.
And given that we are talking about Culebra, that makes it very likely to be andesite or andesitic tuff.
To be sure, it would be necessary to know what minerals and textures it contains, and that would really need analysis using a petrological microscope for starters.
Hi Mike, thank you very much for the answer!
Even under a 10x magnifier I can't see any inclusions in the rock or inhomogeneities of color. There is just a slight porosity on some broken faces. The other faces are as smooth as plastic. Together with the green color, this is what intrigued me in the first place about it.
Thank you again!
A couple of afterthoughts:
- In general, one should also consider the possibility of a specimen like this being industrial slag glass (much of which is opaque). It often has swirls in it, which your specimen does not. And In Culebra, I would think sources for such material are absent (though slag can be brought-in just as a form of aggregate for building purposes, loose paths, etc.).
- I'd go see if you can find some nearby outcrops; see if they are made of similar material. If so, you might be able to find other features, which might allow you to confirm or alter the ID.
i can't seem to get pics to post here's links.
Well, it does look similar to Dan's specimen.
Did it come from the same place?
If not, it doesn't help us with Dan's ID.
In any case, I think it is definitely not slag; I think it is volcanic, and quite possibly andesite. The lack of sedimentary structures and fossils is what leans me away from sedimentary rock, but I can't actually rule that out. As I have written before, it is very difficult to ID fine-grained rocks from hand specimens.
Indeed, it does look very similar. I was wrong to call it "shiny", it has a resinous luster, so it does not sparkle but reflects light like plastic.
For what it's worth, I also measured its density (by the old fashioned method of measuring the volume by the amount of water it displaces), and it is 2.62 g/cm^3. Looking online I see that basalt has density 2.4 - 3.1, and ordinary glass 2.4 - 2.8, so this still does not help for a conclusive identification. Quite a mysterious rock...
Mike, thank you for the andesite identification. I see that there is a "green andesite" variety, but all photos of this I found online show some granular structure. This may be a more homogeneous variety.