Carat

The ring sports three diamonds, each equaling one tenth of a carat.

In the past, each country had its own carat. It was often used for weighing gold. Beginning in the 1570s, it was used to measure weights of diamonds.

A blue diamond, one of the rarest gems in the world, was sold for almost $8 million. That makes it the most expensive gemstone in the world, per carat, sold at an auction.

In addition to carat weight and color, a gemstone's value is based on its clearness and cut—the shape of the polished stone.

It was the star lot of Sotheby's jewelry auction at its annual spring sales in Hong Kong: a flawless, pear-shaped, 72-carat diamond, as big as a walnut.

18-carat gold is 75% gold by mass. 24-carat gold is pure.