Amethyst and Quartz have the same specific gravity so their face-up size
depends on the weight, shape and 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.
2 Ct Amethyst (Cushion)
1 Ct Quartz (Round)
8.08×8.08 mm →
36.96 mm² ← 6.86×6.86 mm
2ct Amethyst Cushion (8.08×8.08mm)
1ct Quartz Round (6.86×6.86mm)
Difference: 59% (21.8 mm²)
2ct Amethyst Cushion (8.08×8.08mm) has approx. 59% more face-up area than 1ct Quartz Round (6.86×6.86mm).