Fibers in a rock?

[Mr.Paarth]

Howdy folks,
New here and had a question about a rock i had found on a camping trip in Montana(couldn't find a spot in the forums for questions like this so sorry if it's misplaced)
We cracked this rock open and the thing had fibers in it. Was wondering if anyone here could tell me what I have here?

 

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[No gold in NY]

I think those fibers could be fine roots that have crept into openings in the porous rock.

 

[Steve1236]

Yup, they're roots, find them like that all the time when I'm chiseling or breaking host rock for fire agates.