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.
4 Ct Amethyst (Cushion)
4 Ct Quartz (Round)
10.18×10.18 mm →
93.14 mm² ← 10.89×10.89 mm
4ct Amethyst Cushion (10.18×10.18mm)
4ct Quartz Round (10.89×10.89mm)
Difference: 0.1% (0.13 mm²)
4ct Amethyst Cushion (10.18×10.18mm) has approx. 0.1% more face-up area than 4ct Quartz Round (10.89×10.89mm).