That's a big stone, no? Was gonna suggest a custom-cut tanzanite but zircon, as @lovedogs suggested above, would be great, too. Or a not-too-dark garnet.
Um, spinel, obviously! For me, it's always spinel, can't help it
This is a great setting, you can really go any way and it's going to look smashing.
What color(s) do you not have much representation of?
I love them, they are the 'everything' stone. Any shade can be found if you look long enough. It's the tertiary color stone. Lucky as well that the most commonly found spinel colors are pinks and purples, you can find some real beauties!
With such a beautiful setting any stone that fits will look great. If you choose a colored stone is there a color or stone that you find that you wear the most?