The largest diamond factory in the US is leading a revolution in the jewelry industry

New York City's Diamond District, located on 47th Street between Fifth and Sixth Avenues, glitters and sparkles, the well-lit diamonds in shop windows luring your eyes in.

But what shoppers might not realize is that behind extremely tight security and a mazes of various locked doors, diamonds are being examined, cut, and polished in the heart of Manhattan's midtown.

Online diamond retailer Ritani is one of the brands whose diamonds are perfected in a factory in the city, and when it comes to carats cut, it's the largest diamond factory in the US. It's operated by the Julius Klein Group, which is a part-owner of Ritani.

Ritani has partnered with roughly 230 local jewelers across the the US and Canada. The brand allows customers to preview their online order in a store, alongside a knowledgeable jeweler who can answer any questions..

This revolutionary "clicks-to-bricks" business model was built with the millennial male in mind. "Not only is this [engagement ring] generally their largest purchase to date, it's typically the millennial male, and he has no idea what he's doing," Ritani's Vice President of Marketing, Mark Keeney told Business Insider.

If the customer likes the ring as they view it in their local store, they can take it home that day - if not, the ring is sent back to Ritani and melted down.

Business Insider recently paid a visit to the factory to see just how Ritani's diamonds are cut and polished.