After much lurking and drooling over all of your goodies, I took the plunge and bought a stone from Tan. It is a peachy pink tourmaline. My question is, it is an oval, size 7.9 x 6.6 x 5.5 mm. What size semi mount should I be looking at?
Well that is one of those sizes that is in between; but I think an 8x6mm 4 prong should work as long as the prongs are rather tall and can be manipulated to open the width as there is where you will have the issues. If you went with a 9x7 to accommodate the width you would have to have the center cut out and that sometimes is a real issue as you do not want a reveal of the basket or head to show around the gem.
If I were you I would take the gem to a jewelry store that has a bench worker and have them set the gem in a 8x6 head and see if it is doable...
I agree with Dana that an 8x6 (if prong set) is going to be your best option BUT it also depends on the setting. The sides of the stone will hang over the setting by .3mm on each side and that might make it look a little odd. If you find a setting, post it up together with the setting and I'm sure people will be able to give you more advice. Luckily ovals are easier to set than other shapes so you've got that going for you!