If I am understanding your pictures correctly, this is under the upper cabinets. If you have to stick your head under the cabinets to see the flaws, you are probably being a little picky. No one will put their head under your cabinets to inspect your backsplash unless you invite them to.

However, if this can be seen by normal height people looking at it in normal ways, then you are right to be more particular about how it came out.

I would think that it would be practically impossible to get a piece of stone to fit perfectly. It's one big piece of a puzzle, and there is no way for it to bend unless it was cut 100% perfectly, and then you would still need room to maneuver it in. So there would always be a gap of some sort, even with a "perfect" fit.

" Am I being unreasonable here? Or is this just the territory that comes with full height stone backsplash? "

Yes. Nothing is perfect. Backsplashes are next to impossible to temple unlike a countertop. As noted above, this is the underside of the upper cabinets and there will be a gap there 98% of the time with full height splashes. The only potential way to do this differently would be to install the uppers AFTER the splash which will bring in a myriad of other issues and may make it worse.

Yes, you are being unreasonable. Gapping is normal. That is small, and to be expected.

Yes, some gapping is normal. Pretty stone. Let them finish the install. If the gapping filled with silicone isn't appealing, then install trim.

Can you take a photo from further back? It looks like you don't have a light rail under the cabinets, which would help to reduce most views of the gap.

The standard bottom alignment of wall cabinets is 54". So, you will need to be shorter than 4'6" to even see that there is a very small, normal, gap.

The same almost 1/4” gap happened when my granite backsplash was applied to the wall after the upper cabinets were installed. The contractor said it was typical and not to worry about it. But I hated it! Thankfully we came up with a solution of 1/2” flat trim to cover the gap all along the bottom of the upper cabinets. Though the cabinets are flat and modern, the trim doesn’t add visual noise yet hides that awful gap. Here are some closeups and one from afar.

I am a fabricator. There is a way to have a perfect fit with full height splash if the cabinets are also new. The technique is to mount the full height splash BEFORE the cabinets are installed. The splash can be in much larger pieces since the installers have vastly superior access. This approach takes good coordination with the job sup but it provides an outcome that is not attainable with the usual process and it typically eliminates 2 or 3 or more seams in the splash.

Ask the fabricator to put a profile on some remaining slab pieces and then cut you a 1" trim.

Under cabinet gaps with backsplash used to really bug me, so I understand your problem with it. However, I have come to realize that no one will see the gap under normal conditions. What really convinced me to lighten up wax when I looked at my own backsplash which I loved. Know what? It has a big gap in places and is crooked. Now I know I am not so picky and it’s. Are it easier for me. Good luck to you!

If the underside of your cabinets are perfect & finished then you might want to caulk the gap but that would be rare.

My full backsplash is darn near perfect; our fabricator is amazing. Even so, they did lay a bead of caulk to fill the very tiny gap. But as others have said, I can't see it unless I bend my head under the uppers and look for it.

A simple little trim would look nice and finished, in the same way a simple crown molding on a ceiling finishes it off.

HU:

Don't use white caulk please. Your gap will nearly disappear with some Axiom Translucent Beige silicone. Faster, less expensive, and more inconspicuous than molding.

Use the caulk recommended above by Mr. Corlett. Tt will erase the shadow line that is drawing your attention to the gap. (LIke Mary's - it looks fantastic) Just a little more finish work and it will be spectacular. You will be pleased.