Other things you want to consider in the setting. (I used google images for sapphire engagement ring and came up with a few different styles)
A solitare?
A solitaire with a diamond on each side
A solitaire surrounded by diamonds? (this was popular style because it looked like Princess Diana's Engagement ring)
[img]ttp://www.faycullen.com/_pics/800/b1003r1s/p.jpg[/img]
or something a little more unique
Some rings will take abuse better than others, she migh prefer a setting that is closer to her hand, rather than sticking out, if she works a lot with her hands..