News flash ... lab-grown diamonds are REAL diamonds and there's really no difference between man-made rocks and their natural counterparts ... according to a famous jeweler.

Jean Dousset, a descendant of Cartier founder Louis Francois Cartier, joined us on "TMZ Live" Friday and schooled us on lab-grown diamonds ... explaining why they are indeed a diamond in every sense of the way.

JD says even the most seasoned gemologists cannot tell the difference between lab-grown and natural diamonds ... and they're both graded on the same scales under the same rules and principles.

It's pretty interesting to consider ... seeing as there's controversy surrounding engagement rings these days. Based on what Jean's telling us here, it sounds like a man would be able to safely pass off a lab-grown diamond engagement ring as a natural one and the woman would never know.

Of course, Jean says most couples are shopping together for rings nowadays and he's not advocating for people to get engaged on a lie ... but the window of opportunity is definitely there.

Jean's got his own jewelry store in West Hollywood and this year he started selling only lab-grown diamonds. He's previously designed engagement rings for Paris Hilton, Eva Longoria and Amy Adams and says couples can get more bang for their buck with lab-grown diamonds.