A huge issue is the deliberate conflation over what a mined vs lab diamond is. Theres lots of interest in keeping the price of diamonds (of all kinds) high, and the best way to do that is to restrict the supply. The language of real diamond/ natural diamond etc is designed to cast lab diamond into this other, un natural, fake diamond role
You can see in this thread that people start talking about cz a lot, or other stones which confuses the issue. Moissonite for example I think is beautiful and offers robustness similar to a diamond, has a cool natural origin of being a space stone, but has a slightly rainbow sparkle. It's also simply not a diamond, in the same way as a Ruby or pearl isn't. I wouldn't chose a sapphire engagement ring because I wanted the hardness, but if I did people wouldn't be telling me it was a fake engagement ring.
I bought a lab grown engagement ring after struggling to get hold of mossionite. Some jewelers were very pejorative over the whole thing for what seemed like little reason. If they had said they were worried about the quality of the actual diamond I would have listened, but most were just telling me it wasn't as romantic, that my wife might feel embarrassed that it wasn't "real". It was very much based on the social shame side of it rather than the diamond its self. One genuinely told me that an engagement ring should show how much you are invested in the relationship and to "go cheap" would suggest I wasn't serious and my wife would feel ashamed of that, and didn't she deserve a "real" diamond like her friends?
There is merit in the argument that they don't resell as well. Because people are put off lab grown by all of the above and jewelers by less of it then the market is less competitive (hence they are cheaper to start with!). However I also know that mined diamonds drop massively in value and second hand you rarely get anything like the value you paid for a bog standard diamond (unless it's a more rare sized one or exceptional one)
My wife didn't mind, I wasn't planning on reselling (I was hoping she'd say yes) and they look and perform identically so while it's come in conversation about the ethical side of it at times, most of our friends don't know and or care. I don't think I'm part of a circle where people care about the clarity and colour of someone else's engagement ring or shame people for smaller rings.
I had a budget in mind, and for the price I could have got a mined diamond, but I went for a lab diamond that was a better cut, clarity, colour etc than the equivilant price mined diamond.
My understanding is that because of the control over the process, mined diamonds are more likely to be higher on the c's as a whole. Mined obviously come in a big range and the low quality ones ones don't come to market as individual stones, and others with lower clarity for example are used in other forms of jewellery, studding etc and the smaller percentage of higher quality come to the jewelers.
When I went to the jewelers to have the ring made, they showed me a great big list of all the diamonds I got buy for that fitting, where they rate them by size, cut, clarity, colour. If you want a better clarity then you'll pay more, or you can move down the scale on size etc. Like booking a holiday where you can are combining price, hotel and number of nights. If you want to a nicer hotel for the same money you have to compromise on the number of nights. With lab grown its like looking at a cheaper destination, where you get more nights for your money.