You won't believe how much some diamonds are selling for.

"Colored diamonds used to make a $1 million per carat, which was a great price and today, $4 million a carat is the new normal," said Rahul Kadakia, head of jewelry at Christie's International. "We've seen prices for white diamonds at over $250,000 per carat, which is a price that's five times higher than 10 or 15 years ago."

On May 18, Christie's is set to auction a rare blue diamond in Geneva, Switzerland, that could fetch more than $45 million.

"At the highest end, there's a great shortage of fine gem stones, fine jewelry, fine colored gem stones and there's a big group of buyers out there who want to buy the very best, which is what's driving these prices."

While Kadakia said the demand for these stones changes based on different economic cycles, it's consistently strong among buyers in certain areas.

"You always have Asian clients, who have been big connoisseurs of great gems," he added. "The Middle East has constantly been buying with us. Russia has been buying lately and established collectors in the U.S. and Europe who are always there."

Since the start of the year, economists have been worried about a global recession. China, the world's second-largest economy, is undergoing a massive economic transition that is set to bring about lower growth. During first quarter, its economy grew by 6.7%, compared to 6.9% for all of 2015.

But Kadakia isn't worried about slower growth in the world's biggest economies.

"Things can't keep going up forever," Kadakia said. "They have a little dip, they correct themselves, and they come right back up, and we saw that when the market got a little quiet in 2008, but it bounced back very quickly in just about a year."

Within the broad diamond market, Kadakia said colored diamonds are performing "extremely well."