5ct Moissanite (Emerald, 11×9mm, White) vs 5ct Sapphire (Emerald, 11×9mm, Blue)

All things being equal, Sapphire will face-up smaller than Moissanite for a given carat weight, because it has higher specific gravity (4 vs. 3.22 - this means it's "heavier" compared to Moissanite of the same size).i Face-up size also depends heavily on the proportions of the cut.

Face-up area is a measure of the size of the gem when viewed from above (as set in a ring). It tells you how big the gem is at the girdle plane.