Quartz and Amethyst 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 Quartz (Round)
1 Ct Amethyst (Round)
6.86×6.86 mm →
36.96 mm² ← 6.86×6.86 mm
1ct Quartz Round (6.86×6.86mm)
1ct Amethyst Round (6.86×6.86mm)
Difference: 0% (0 mm²)
1ct Quartz Round (6.86×6.86mm) and 1ct Amethyst Round (6.86×6.86mm) have the same face-up area.