3ct Topaz (Round, 8.99×8.99mm, Blue) vs 1ct Emerald (Emerald, 7.28×5.39mm, Green)

All things being equal, Topaz will face-up smaller than Emerald for a given carat weight, because it has higher specific gravity (3.53 vs. 2.72 - this means it's "heavier" compared to Emerald 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.