It doesn’t matter what it is. If it’s epoxy fill, that’s stronger and more durable than the stone. If it’s a natural fissure or vein, once installed, it’s fine as long as no one dances on the counters.

All exotic slabs come to market with repairs of some kind. That’s an unimportant red herring . If you love it, buy it. Just buy an extra slab if you want the best seams. A standard 2 slab kitchen needs 3 slabs with movement like that and corners.

I’ve dealt with stone for over 25 years as a designer. Before a fabrication shop littered every corner, and when it was just starting to hit popular mainstream use. I don’t tool it, or look at it under a microscope. But you can be certain that I’ve seen a thing or two. This is a non issue. Complete non issue. If you love it. If you don’t, move on.

Thank you! Fortunately we don’t have corners to deal with!

Here's a link to a helpful article about being your own stone sleuth. And here's a post from karin_mt, a geologist and contributor here. Her post has info about doing a scratch and etch test, plus links to other threads on counter stone topics.

Look at the area in question under low level light. It is difficult to get filler to have the same gloss level as natural stone.

This article talks about how to tell the difference between a crack and a fissure. If you run your finger over the crack and if it slides across smoothly it would indicate a fissure and if there was an interruption that would signify a crack. However, according to this article some fissures can eventually turn into cracks so that would be something to consider. The uneven edges on this fissure/crack would give me pause and I would visit other stone-yards and compare a number of slabs to see if they all share this similar feature.

https://www.hardrockstoneworks.com/blog/20/the-difference-between-cracks--fissures

It’s a vein. Forget it.

I guess the combination of the above responses, from don‘t worry about it to worry about is my problem. Now I’m right back where I started. Do any of you work with stone on a daily basis? Thanks.

@Granite City Services is a fabricator, and @karin_mt is a geologist. Both have provided really helpful info on this forum so I'm tagging them in the hopes they'll see this and chime in.

"Do any of you work with stone on a daily basis?"

Yes. We repair stone and diagnose problems every day. I wrote a column on countertop repairs for four consecutive years. My work has appeared in Old House Journal, Solidsurface magazine, and The Journal of Light Construction. I used to have 3 licenses in 2 states, but I let Michigan lapse. I'm a member of Ted McFadden's nationwide stone repair network and he doesn't let just anyone in.

Thank you for the helpful suggestions! I forgot to mention that the same line is on two slabs. I’ll bring your notes with me when I look at the stone again on Tuesday.