It's cut like a diamond, it shines like a diamond and, essentially, it actually is a diamond. But it didn't form underground.

There's now a cheaper, more ethically sourced option for diamond lovers – though not every diamond retailer is sold on the idea. Synthetic diamonds, which are grown in a laboratory using advanced technology, have the same chemical composition as a traditional diamond, but leave a little more cash in a shopper's wallet. Lab-grown diamonds are just as real as mined diamonds, but take about 3 billion less years to form, give or take.

These man-made gems are still a niche product in the jewelry industry, but some experts expect sales to ramp up within the next few years. According to Morgan Stanley, sales of lab-grown diamonds make up just 1 percent of the global market of rough diamonds.

Amish Shah, the president of ALTR Created Diamonds, which produces synthetic diamonds, said the category's estimated market value stands at $150 million, but could grow to $1 billion by 2020.

One factor playing in favor of created diamonds is shifting consumer attitudes. Millennials have shown less desire to buy mined diamonds than previous generations — many say they'd rather save their money or purchase a gem that was produced with sustainability and ethics in mind. Traditional diamonds must be mined from the ground, which can have a negative impact on the environment such as soil erosion and deforestation. Not to mention, that many diamond mines are in conflict zones, and consumers may worry about mining profits supporting violence in the region.

"For customers that are seeking to minimize the environmental impact of a jewelry purchase, lab-created diamonds are an ideal eco-friendly choice as they require no mining," said Kathryn Money, the vice president of strategy and merchandising at Brilliant Earth, a retailer for ethically sourced fine jewelry.

Also, synthetic diamonds generally costing about 30 to 40 percent less than mined diamonds.

About 70 percent of millennials said they would consider buying a lab-grown diamond for an engagement ring center stone, according to a study by MVI Marketing. Meanwhile, searches for lab-grown diamonds on Google have almost tripled in the last decade, according to Money.

That's a big shift. Consumers have generally been more accepting of other types of synthetic gems. According to the Gemological Institute of America, other lab-grown gems have existed for decades, but they've had little impact on sales of natural gems.

"The market for natural colored gems (ruby, sapphire, emerald, etc.) has co-existed with synthetics for more than a century," a representative for the institute said in an email. "Demand for natural rubies, sapphires and emeralds remains as strong as ever and synthetics have evolved into a separate market. It is possible that the markets for natural and synthetic diamonds may develop in the same way."

Even though lab-grown diamonds may not be a threat to the mined diamond market, some jewelry retailers still believe that natural diamonds are the best choice for consumer to make.

Signet Jewelers, the world's largest retailer of diamond jewelry, which owns companies like Kay Jewelers and Zales, does not sell synthetic diamonds, though it does sell other synthetic gems like sapphires and rubies.

David Bouffard, the vice president of Signet Corporate Affairs, said Signet's research "shows that our customers, including millennials, prefer natural over synthetic diamonds, especially as it relates to bridal jewelry."

Signet is a founding member of the Responsible Jewellery Council, which develops key standards for the jewelry industry. The group says it is essential that lab-grown diamonds are labeled so consumers are aware of the product they are buying.

But supporters of lab-made diamonds insist these gems are the real deal.

"Created diamonds are real diamonds," Shah said. "They are just as real as the beautiful flowers from the greenhouse."

Lab-created diamonds can be made two ways: the high pressure, high temperature (HPHT) and the chemical vapor deposition (CVD) methods. According to the Gemological Institute of America, the HPHT method mimics the high pressure and temperature conditions that form a diamond underground. In these conditions, carbon atoms build on a tiny man-made or natural diamond seed.

The CVD method is done by breaking down carbon-rich gas in a vacuum chamber. The gas breaks down into carbon and hydrogen atoms, which are depositing and build on a diamond seed.

Money said there have been recent advancements in lab-made diamonds, which can now be made larger and of higher quality. To the naked eye, synthetic diamonds can be of such high quality that they can't be told apart from traditional diamonds. Instead specialized instruments would need to be used.