Where did I go wrong?

[Roly]

I believe you are experiencing normal seasonal movement of the cherry wood.  The wood is shrinking with the lower humidity of the winter season but the quartz is not shrinking.  Something had to give.


Mike

It will help if you have the grain going lengthwise on the ends.  There will be little change in the length of the boards but most of the movement will be across grain as in the end pieces.  But it is best to have a small gap between dissimilar materials even with a change in grain direction.  Silicone caulk will allow some movement.   Roly

[ianab]

Route the gap around the quartz at least 1/16" larger all around and then caulk it with silicone.  That will allow the wood to expand and contract w/o cracking.  The rest of your plan is good. 

John

This ^, or some variation of the idea. 

The wood is shrinking as it dries out in the heated winter environment. When one side wasn't finished, you noticed the effect first as warping. The unfinished side changes moisture level faster than the varnished side. Adding moisture evened the wood up, and now you have a similar finish on both sides (which is a good thing). But quartz doesn't shrink at all with changes in humidity... So you need an expansion gap, and you could fill it with something flexible like silicone or rubber beading. 

It's like steel road bridges. They have some sort of expansion gap that's usually got a rubber block in it. In that case the bridge is moving because of changes in temperature, but the idea is the same.

[handi]

“If you can’t hide it, flaunt it.”

You cannot have no gap, so make the gap a feature. This is why quirks and set backs were invented, to hide errors. 

With my dining table, I knew full well that the seam between the breadboard ends and the top would never remain perfectly aligned and flush as the seasons changed. So I added a small “V” detail along the seam. Just 1/16” deep and 1/8” wide. Looks good and fools the eye about any variation in the seam over time.

With this project, I’d be tempted to make the Pocket for the tile 1/8” oversized all the way around, stain or paint it black, then bond the granite in the center using silicone. The wood will be free to move, and no one will be able to tell by eye if the gap is 1/8” or 3/32”.

[mdhills]

To work around wood movement, I think you could also set the stone in a mitered rectangle (add splines for strength).

[BrentDH]

If you were willing to not have the granite flush with the wood you could create a rabbeted opening and mount the granite from the underside.  Maybe have a 1/8" thick lip that covers the edge of the granite for 1/4" or so.  Then the required gap between the granite and wood would only be visible from the underside.  Maybe use the bead of silicone as mentioned to hold the granite into the rabbet.  I would not even try to get the silicone into the gap - just make a bead to hold it in place.

Obviously the cherry would rest directly in contact with the top of the granite but this was the best I could do with ascii characters.

                                    _____________________
                                   /__
-------------------------------  |
granite                            | |             cherry
-------------------------------  |
                       silicone    (  |____________________

[AHill]

Agree with the seasonal movement of the wood. It'll expand and contract perpendicular to the grain direction. The quartz pretty much stays the same. In the winter, it's likely contracted, and the quartz said, nope, I ain't movin'. So the gap will allow the wood to expand and contract without being restrained by the quartz insert. Also, how dry was the oak when you built it? If it wasn't acclimated to the room before you started cutting, or it wasn't properly dried, there could have been residual stresses that cause the warping and/or exacerbated the cracking. If it was kiln dried, then you need to give it a couple of days or more in your shop to acclimate prior to milling it.

[TomFromStLouis]

Quartersawn wood is much less prone to cup and warp as you show. You still need to deal with wood movement to a degree, but the origin of your problem - wood wanting to move - is much reduced.

[John Mihich]

“If you can’t hide it, flaunt it.”

You cannot have no gap, so make the gap a feature. This is why quirks and set backs were invented, to hide errors. 

With my dining table, I knew full well that the seam between the breadboard ends and the top would never remain perfectly aligned and flush as the seasons changed. So I added a small “V” detail along the seam. Just 1/16” deep and 1/8” wide. Looks good and fools the eye about any variation in the seam over time.

With this project, I’d be tempted to make the Pocket for the tile 1/8” oversized all the way around, stain or paint it black, then bond the granite in the center using silicone. The wood will be free to move, and no one will be able to tell by eye if the gap is 1/8” or 3/32”.

I often do that.