I really like the idea of vines and flowers for the border. Perhaps use just the colors in the vines and flowers that you already have in the quilt. That would complement all of the angular lines in the blocks and not add any more angular lines. I like your quilt and the colors you used. It is very nicely done. I would bind it in the darker blue if possible (meaning that you have enough of it.) I will look forward to seeing your final decision.

How about a 1-2 inch white border and then the 6.5 inch border with the corner stones?
You could use a medium blue similar to the blue in the body of the quilt.

I can hardly believe seeing your new thread. Just yesterday I finished the exact same mystery quilt with my guild in Calgary! Mine is finished a year late, but that's beside the point :) I too was stuck on the borders wanting to use up what I had already used in the quilt. Here are 2 photos of what I ended up doing. I hope it gives you an idea. I love the colours you used. Aren't mystery quilts just amazing when seen side by side?

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Also, I flipped the border corner blocks because I thought it would be very difficult to make the points (the 3 little white triangles) nice when attaching a binding over them.

Melanie,
I just commented on your post about finishing this mystery quilt a year late, that it was like mine. Then I come here and see you noticed the same thing! I chose this finish for the outer borders because it used fabric I had. I turned the cornerstones from the directions my pattern said to do them because I liked how it looked like this better.
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Your border turned out great! I know what you mean, I really wanted to use as much fabric as possible that I already had in the quilt. The small squares that you used compliment the quilt pattern perfectly.