10ct Beryl (Trillion, 17.26×17.26mm, White) vs 10ct Diamond (Trillion, 16.67×16.67mm, White)

All things being equal, Diamond will face-up smaller than Beryl for a given carat weight, because it has higher specific gravity (3.52 vs. 2.72 - this means it's "heavier" compared to Beryl 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.