Doggramma

I like your idea. Your quilt is lovely so far

Cedar

I love the braid idea! I think it will finish the Celtic feel perfectly. It will be tricky to keep it from overpowering the simple elegance of your stones. I would choose a green that's similar in value (lightness) to your tan and similar in tone (yellow/blue content) to the greens in the field, allowing the Celtic intricacy to fade into the background a little. I'd make sure that the beige is darker than the lighter parts of the stone so that the white of the stones draws your eye strongly.

Kat2

Love the quilt. Love your idea. But also what comes to mind is since it is a picture to use a wood grain fabric (maybe 2 different shades, for light and shadows) to frame the quilt. If you do use the tan or beige, see if you can use one with a little directional and do mitred corners.