De Beers has a way of monopolizing markets, so it may be difficult to find a provider of an unstamped synthetic.

No, it won't: https://www.alibaba.com/countrysearch/CN/cvd-diamond.html

“Monopoly” doesn’t mean there are literally no alternatives.

However De Beers aren't the only ones who know how to make them, and given their apparent distaste for them they probably don't hold the patent.

Unless they intend to buy every company that starts making them or pay the patent holders a fortune, I don't see how they can effectively monopolise this like they can real diamonds.

"De Beers said the logo won't be visible to the naked eye"

I think what they is saying is that a jeweller could get synthetic diamonds from another source for the same or less than Lightbox's marked diamonds and on-sell them as real. Only the original jeweller would know they aren't real, especially if the manufacturer can introduce random impurities.

What do you mean by "aren't real". In what way is a manufactured diamonds not real?

My understanding is that they are in every respect indistinguishable except for perhaps being too perfect under magnification.

In the same way that a fake Picasso isn't real because Picasso never made it, a lab grown diamond isn't "real" because the earth didn't make it.

What makes a Picasso painting a Picasso is that it was made by him. What makes a diamond a diamond is its molecular structure.

Following this logic sea salt isn't "real" salt like regular table salt (which is mined), since it wasn't mined from the earth, but created from sea water.