Rock id of stele's fragments.

its really hrd to tell anything. Even the inscriptions on the stele are un recognizeable. The second one, what with the chicken wire shadow, even looks like a concrete slab

I am almost sure that is not concrete, unless not concrete I am used to see. The are two types of carvings on it: sunken relief (numbers) and outer relief (letters and map). I can read and reconstruct the inscriptions readings except for the top's one which is incomplete [...uim(l\t)a] and seems not to be spanish. I could discern two numbers which I infere, are the date [18-64]. Which would make sense because I don't remember seeing inscriptions of this type today and maybe there is nobody who carves stone today in the region nearby. I don't say there was not concrete in XIX century, because it was produced since the Romans. But even the Romans used to carve their public inscriptions on stone. When I was carrying one of the fragments a chunk drop off from the reverse. I will try to get better photos. Thanks.

take a small chunk and immerse it in table vinegar and tell us what it does.

I immersed it a piece of it in vinegar but nothing happened, it just got wet. I took a close up pictures, I noted it has mica and greenish feldspar or quarz inclusions. What could it be?


https://www.flickr.com/photos/51986587@N04/14748893171/sizes/o/
https://farm3.staticflickr.com/2921/14771917633_358b9c05f6_o_d.jpg

from your new photo, it appears to be a leucogranite (light colored), with the green minerals probably a uranium mineral like autunite. Wed call the rock an Alaskite probably. the thing with rocks, especially igneous an metamorphics, the basis for thwir classification is the texture and the realm of feldspar, type of mica, associate minerals and the ratio of quartz to everything else. SO, an alaskite is a guess but its probably close enough without doing more microscopic (thin section staining)
AN alsakite is one where quartz is the greatest concentration and the feldspars are greater in prcentages than feldspathoids, and the feldpsars are probably sodic or potassics (like albite, orthoclase, microcline or even amazonstone)
The greens can be the amazonite or the autunite and the micas are probably the bronzy kind.

Its a guess but alaskites often harbor some neat mineral associations like Tourmalines

I know granite by itself is lightly radioactive, but how radioactive is autunite?

it could be the autunite or a form of microcline. If the green rock has a "cleavage" on 2 sides, then its a microcline. If it just breaks into a hackly structure it may be autunite.
Autunite is as radioactive as an old watch dial. You don't want to give it to kids to play with or would you make jewelry of, but on a shelf, no problems.
Many rocks, as you already stated, are milly radioactive. Autunite is just one where it can be used as an ore if its in high enough concentration.