What specific material is the box made from? If ply, what type, thickness, and brand? What type of joinery holds the box together? How is the box hung on the walls? Using what? There are no mechanical fasteners holding the quartz to the box? Just adhesive? What type of adhesive?

There is no room for error here when having 1000 pounds above your head. The weak link is the box material, how it is joined, and how it is fastened to the walls. Adhesive can also be a weak link without additional mechanical studs into the quartzite, or a L channel as support.

I would want some sort of mechanical fasteners to secure the stone in place. It should not be a problem, but wear a hard hat and steel toed shoes when you cook. Make sure your cabinet guy's and fabricator's insurance is always paid and up to date.

It's definitely gorgeous! It's after the fact, but still I'd do some research on how your inspiration hoods were constructed. Here's an old thread (link) from a forumite who did a very large marble range hood. The thread shows pictures of the framing used to support the stone. It was no small feat.

You are wise to be concerned. Google says quartzite is about 2700 kg/m3, so it looks like you have about 300 lbs of stone above head height - easily enough to be lethal if it fails.

I would also want mechanical fasteners between the slabs and the structure underneath, and I would want to see the structure underneath including how it is attached to the studs. Can you open up the wall on the other side to take a look? Or remove the insert?

Short of dismantling it, there is no way to verify how much epoxy is contact with both the plywood and the stone, so even knowing the epoxy’s shear strength wouldn’t tell us if this is safe. So again, I would advise mechanical fasteners (bolts, presumably - though I would then start to worry about tear-out in the plywood; I would hope a professional would have a good solution but if that expertise were on hand you wouldn’t have had to post here)

The chances of this being lethal are slim. Even if it let go, it would probably bind between the cabinets well before hitting the floor. I would do a bit more non-invasive research and base my worrying on that.

@Joseph Corlett, LLC the front slab (150 lbs or so) would be my main worry.

The fact that ther is no L channels at the bottom, or throgh material bolts of some kind, and no one knows how anything is mounted to the wall, or constructed? That would give me a whole lot of concerns about a whole lot of the build out.

Im an engineer and there isnt enough info to give a definitive answer.

You need to know specifics of:

What plywood is the box made of.

How are the box joints held together

What is the box mounted to on the wall

What fasteners are used on the box

Are there any brackets/screws/etc for the stone that are hidden behind the trim pieces

What epoxy was used

What is the total weight

The contractor should be able to easily answer these questions. If they give you the run around, that would be a huge red flag to me.

where did you find the 3/4 slab ? it is goergous