Carat Weight (size is not weight)
Diamonds are rare.
The rough diamond in the centre could be cut to either the sparkly 0.80ct stone on the left, or the dull drab 1.00ct stone on the right! Weight does not equal size; both diamonds have the same diameter!
Carat describes weight: 5 carats = 1.00 gram. Carat does not describe size or measurement. Weight is crucial to cost; doubling weight quadruples price. More than 90% of all diamonds are smaller in diameter size than they should be. Well cut 1.00ct round diamonds should measure more than 6.4mm, but most carat stones are smaller than 6.3mm or 6.5% bigger surface area or ‘spread’. Smaller diameter deeper diamonds sparkle less; poor light return means they look even smaller. Why is this so common?
Cutting diamonds to save weight meets consumer demand for ‘magic’ carat weights. Sparkle is sacrificed because people want 1.00ct and 2.00ct diamonds as you can see from the chart. Cutters profit at the expense of diamond beauty. GIA, the main industry gate-keeper, is powerless because measuring sparkle is too hard. I have crusaded against poor cutting practices for decades with quite a lot of success.
Consumers pay more for ‘magic weights’. Cutters cheat to meet demand and get a higher price per carat. Cut quality and beauty suffer.
TAKEAWAY: most diamonds are cut to save weight. Diamonds of twice the weight cost 4 times more.
Carat weight costs way more than Cut quality because it's really hard to grade Cut.