I am in the same situation - grey cabinets and black quartz counter. Dal-tile has a nice one I'm considering called Exquisite. It comes in different tones.

Look at this...

Haha they are untied. I was experimenting lol. Those are beautiful! Tiles?

Here's how to arrange it on the wall around the window.

what color is your counter top...really pretty...First I would get rid of the valence it is making the window look smaller..but the herringbone patter tile is beautiful that someone posted here, but I think that's carrera marble...

Your kitchen is going to be beautiful. Marble herringbone pattern two above would look fabulous with your cabs and counters.

You could raise the valance over the sink window, not by hemming it, but by moving the rod up to open it up a little more.

I love leelee's choice of tile. I would actually get new window treatments as the colors are drab. Do you happen to have paint left over from the island? If so, it would look better to paint out that vent to make it disappear;)

When looking from the kitchen into the rest of the room I see a ton of tiny things on the walls. It gives your room a busy, not together look. Try removing all the snapshots and create a hall gallery for them. They aren't going to compliment your fabulous kitchen. Remove everything and start over adding just a few larger prints.

Jeff Lewis has a paint color called "Dirty Martini". Match the tile to that and you're good to go. You're already warm with the ivory-color cabinets. If you try to go too neutral, it doesn't work (your gray wall already isn't working). So, go cool instead for a nice contrast.

Plain and simple stainless steel sheets available at Lowes and Home Depot. They come in square and rectangle . Easy clean up , looks modern, stone is nice but keeping it clean will be difficult. Unless you never have pasta sauce, bacon or gravy splashes to worry about. The Stainless backdrop will really make your cabinet stand out.