When Calvin Mills Jr. decided to propose to his longtime girlfriend, he visited countless stores and spent hours scouring websites in search of the perfect diamond ring.

The technology expert knew he wanted something special to impress his intended fiancée, Brittany Ramsey, but the prices were giving him sticker shock.

Then he stumbled across Pure Grown Diamonds, a new company that manufactures its gems in a lab.

While the synthetic diamond industry has existed for decades — think of all the cubic zirconia sold by the crateload on QVC — recent advances in technology have led to a boom in high-end, lab-grown gems.

About 10 companies now produce them, with most suppliers to the US based in the Far East, India and Russia.

“These are the diamonds of the future,” says Lisa Bissell, Pure Grown Diamond’s CEO. “They are revolutionary because they’re sustainable and conflict-free. They’re a whole new choice for eco-minded consumers.”

According to Bissell, about 30 retailers in the US have begun stocking Pure Grown Diamonds within the past two months. The company claims their gems have the same composition, color and clarity as traditional ones — and they cost up to 50 percent less.

“It was a no-brainer to me,” says Mills, 37, of Baton Rouge, La., who proposed to Ramsey during a football game Thanksgiving weekend at his alma mater, Southern University. “The diamonds are not only beautiful, they’re ethical and, I guess most of all, affordable.”

Although he prefers not to reveal the exact price he paid — “I’ll put it in the $20,000 to $40,000 range,” he laughs — Mills believes that the 2.62-carat pear-shaped yellow diamond he purchased was worth every penny.

And Ramsey concurs.

“I absolutely love my ring,” says the 30-year-old public advocate. “Everybody has been going crazy over it.”

But the diamonds are not without their detractors. Conventional jewelers are understandably concerned about being undercut, while traditionalists argue that lab-made gems just don’t have the romantic associations of a mined stone.

“Ultimately, when a man buys a ring for a woman, he’s buying a diamond because a diamond is rare — it’s precious,” says Zameer Kassam, who runs an online fine-jewelry business.

Instead of being formed over millennia below the Earth’s crust, it takes about six to 10 weeks to produce a lab-grown diamond by blasting carbon fragments with heat, chemicals and gases that replicate the natural process. (The official term for this is “microwave plasma chemical deposition.”)

“Yes, they’re becoming a major phenomenon and, to the layperson, it’s really hard to distinguish between lab-created and mined diamonds,” says jeweler Adam Gil, who runs his family-owned business, Jerry Gil & Co., out of the Diamond District.

But, while he thinks the artificially grown gems might become popular in the flyover states, in the case of New York, not so much.

“I really don’t think lab-created flies with the New York crowd,” says Gil. “As an Upper East Sider, I can safely say that girls really want the real thing.

“As the saying goes: ‘Diamonds are a girl’s best friend’ — not lab-created diamonds.”

Still, Stacy Tasman, of the engagement website HowHeAsked, says the new high-tech bling is not without its appeal — especially given concerns over “blood diamonds,” the environmental impact of procuring a gem and, most of all, prohibitive costs.

“But I think it’s going to be a hard struggle to win over a certain sector of the market,” she maintains. “There is this novelty factor of having a diamond from De Beers or Tiffany, and the desire to have a real diamond is ingrained into the culture here in the US.”

Asked whether she minds that her ring was made in a laboratory, Ramsey claims it simply makes it extra-special.

“I’m flattered that Calvin put all that time into researching the background and origins of the diamond — something which many men wouldn’t even consider,” she explains.

And, if it cost a little less than a mined diamond, all the better.

“I’d rather put that money toward our future together.”

How lab-grown diamonds are made

â–  Lab-grown diamonds are cultivated from the tiny carbon seeds of pre-existing diamonds.

â–  The seed is placed inside a low-pressure microwave chamber.

â–  Hydrogen and methane gases are then introduced.

â–  A microwave generator pumps energy into the chamber that ignites a glowing plasma ball.

â–  Carbon molecules rain on the seed, layering on top of it and fast-tracking the natural crystallization process.

â–  The process is completed within six to 10 weeks.

â–  The diamond is cut with a laser, polished and then certified.

â–  To prevent fraud, the near-gem has the words “Lab Produced” lasered on the underside — a code invisible to the naked eye that can only be seen through a microscope.