For many couples who recently cemented their commitment to each other with an engagement, the choice of ring featured not natural but man-made gemstones — including the center diamond.

While a diamond continues to be the most popular type of engagement stone, nearly one in four engagement rings in 2021 featured a man-made center stone, not necessarily always a diamond, up 11% in the past two years, according to a report from wedding planning website The Knot."

The popularity of lab-made diamonds is growing because of the eco-conscious mindset of Millennial consumers and a subset of GenZ-ers, said the report, which was based on a survey conducted in November of 5,000 US couples who became engaged between January and November 2021.

Another factor fueling the preference for a synthetic diamonds: They're less expensive than mined diamonds. They can cost as much as 30% less, said Shelley Brown, The Knot's senior fashion and beauty editor. Lab-made diamonds and other gemstones have pushed into the mainstream in the last four to five years, she noted.

Lab-made goes mainstream

Leading jewelry retailers are also driving that effort.

The True Vera Wang LOVE collection for Zales has 16 engagement ring styles that feature lab-grown diamond center stones.

This May the world's largest jewelry company, Pandora (PANDY), made a major shift by announcing it would stop using mined diamonds in its jewelry.

Instead, the Copenhagen-based company is shifting to lab-created diamonds, which it said have the same "optical, chemical, thermal and physical characteristics" of a mined diamond and are graded by the same standards known as the 4Cs: cut, color, clarity and carat.

Pandora said it's instituting the change as part of its own effort to sell sustainable jewelry, and also because consumers are asking for it.

Signet (SIG), the largest jewelry company in the United States, is stocked up on engagement rings.

"This is the high season for bridal jewelry. There are a tremendous number of couples who get engaged from Thanksgiving through to the New Year, and we're ready to go," said Jamie Singleton, president at Signet Jewelers, which owns Zales, Kay Jewelers and Jared.

Singleton said three big trends dominating engagement ring preferences include larger stones of 1 to 3 carats, yellow gold and fancy center stone shapes like oval, pear and emerald.

She also sees the growth in demand for lab-created diamonds.

"This is very Millennial-based, and frankly they are the demographic that represents most of the shoppers in the bridal category anyway," said Singleton. "The lesser cost of a lab-created diamond allows couples to buy a larger stone."

Currently, 4.7% of the specialty diamond jewelry market in the United States is represented by lab-grown diamonds. That figure is up a whopping 34% from 2020, said Edahn Golan, an industry analyst and founder of Edahn Golan Diamond Research & Data.

"Regarding the cost difference, a 1-carat engagement ring with a lab-grown diamond can cost 60% less than a 1-carat natural diamond ring," said Golan.

The new "LEO Legacy lab-created diamond" collection is a collection of 21 engagement rings and bands at Kay Jewelers.

Although a majority of shoppers at its stores are still buying natural diamonds, Singleton said Signet has expanded its man-made bridal jewelry selection this year. This includes a new "True lab-created Diamonds by Vera Wang LOVE" collection for Zales, which includes 16 engagement ring styles that feature 101 facet lab-grown diamond centers, as well as the new "LEO Legacy lab-created diamond" collection featuring 21 engagement rings and bands at Kay Jewelers.

But there is one important consideration for anyone buying lab-created diamonds: their resale value.

"A lab-created diamond is really not as big of an investment as a natural diamond," said Brown. "Consumers may not be educated about this."

Martin Rapaport, founder of the Rapaport Diamond Report and chair of the Rapaport Group, agreed.

"Synthetic diamonds are not subject to natural scarcity like a mined diamond. They can be produced in unlimited quantities by machines," said Rapaport. "So they don't really retain a resale value like natural diamonds."

"I think it is misleading to sell man-made diamonds without this important disclosure even if some consumers may not care about the resale value," he said.

This article was written by Parija Kavilanz, CNN Business.