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.
1 Ct Amethyst (Cushion)
1 Ct Quartz (Round)
6.41×6.41 mm →
36.96 mm² ← 6.86×6.86 mm
1ct Amethyst Cushion (6.41×6.41mm)
1ct Quartz Round (6.86×6.86mm)
Difference: 0.1% (0.02 mm²)
1ct Amethyst Cushion (6.41×6.41mm) has approx. 0.1% more face-up area than 1ct Quartz Round (6.86×6.86mm).