The round brilliant cut was patented in 1919 by Marcel Tolkowsky and has been refined every decade since. It uses 57-58 facets cut at very specific angles to maximize the return of light through the top of the stone. No other diamond shape returns as much light to the eye, which is why round brilliants account for about 70% of engagement rings sold.
The popularity has a price. A round 1.0ct G/VS2 retails for $4,500-$6,000. The same color, clarity, and weight in oval, princess, or cushion costs 12-18% less. You're paying for the optics — and for the fact that round is the only shape GIA grades on cut quality.
When round is the right call: traditional taste, maximum sparkle, easy to find, easy to resell. Round retains value better than fancy shapes because demand is steady and predictable.
When round is the wrong call: you want a finger-coverage look on a budget. A 1.5ct oval or pear visually reads as larger than a 2.0ct round, at much lower cost.