The Carat System
How did the carat system start?
The modern carat system started with the carob seed. Early gem traders used the small, uniform seeds as counterweights in their balance scales. The carat is the same gram weight in every corner of the world.
What are “magic sizes”?
For diamonds that are equal in every way, diamond value increases as weight increases. At certain weight boundaries, called “magic sizes,” the price per carat increases significantly. That’s because these boundaries or thresholds coincide with popular weights, and popularity means greater demand and therefore higher prices for these weights. This is especially true at the magic one-carat size.
A quick comparison of two diamonds shows how magic sizes can affect price. If one diamond weighs 0.96 ct and another weighs 1.02 ct, the 6 pt (0.06 ct) difference in size is almost imperceptible. But if both are D-color round brilliants with identical clarity and cut, the difference in cost is significant. The fact that the second diamond is slightly over the “magic” one-carat threshold may cause its price to be as much as 20 percent more.