There is no simple answer, if in doubt have the gem inspected by a competent jeweler. Good diamonds are pretty, even pretty good diamonds are pretty, and if it's ugly, it's not pretty. The round diamond is the most frequently chosen and technically the most efficient in producing the diamond sparkle. Other shapes including the marquise, pear, oval, emerald, princess triangle and others vary in popularity but can often permit spectacular designs.
Taste usually determines the choice of metal for jewelry.
From a technical standpoint each has unique advantages that may best suit a particular design. Rely on your taste for color and a skilled jeweler can best guide you in choice of metal. You should have no hesitance in judging the first two. Rarity however, is where you'll have to depend on a skilled professional. When a hundreth of a carat in weight or an almost imperceptable color grade difference means thousands in market value, you need expert advice.