As important as finding the perfect Vanna K semi-mount is finding just the right diamond to go with it.
The shape is often the toughest choice, and there is really no exact formula. In the end, it's really up to what you personally like. Marquis cuts are somewhat out of favor these days, a fact you may find of interest if you're fond of running against what's currently "in", but Princess, Round, Oval, and Emerald diamond rings remain strong sellers.
Then there are the aspects of the diamond less obvious to the eye, but critical in determining level of dazzle, and value in the marketplace. The Gemological Institute of America has dubbed these the "4 Cs" - carat, clarity, color and cut.