When choosing the shade for a feldspathic porcelain crown, which component is most important?

Leucite crystals scatter and refract light within the porcelain, and their size/distribution strongly influences the optical properties (translucency, opalescence, value) that determine how a feldspathic crown's color and shade are perceived — making them the most shade-critical component. Feldspar is the base glassy matrix, and quartz contributes mainly to strength/structure rather than shade.