May we see the rest of the room so we can make suggestions that make sense with the rest of your decor? Just comment here and use the Attach Images button to show us the rest of the room and your furnishings.

Artwork would be my suggestion. I've made general suggestions below but it would be best to see more of the architecture and decor before offering more suggestions.





Most of the above came from my "Architectural Accents" Idea Book.

Can you get stones to match and fill it in?

Subtle art. There is already a lot going on there

I think I would look for a piece of art like this. My aunt once had this big, bronze Aztec sun dial medallion, and it seems something like that would fit nicely in that niche, and fit in the with rustic decor. You also wouldn't have to worry about it very much, just find some way to dust it occasionally.

An oversized frameless pendulum clock would be nice too. Backlight stained glass could be stunning too. Here's a clock idea but It would need to be bigger:

Thank you for your suggestions. I like the subtle artwork idea, a decorative frame, and love the tile and back lit stained glass but not sure how feasible. Thanks much. I love Houzz.