What is? The certificate? Because otherwise you wouldn't know. For the same value of 'how much it cost', a lab-grown diamond (we're not talking about other non-diamond synthetics) will look better (clearer, fewer flaws) and/or be bigger. If you want to show it off on your finger, you want lab-grown.
Sure you can just lie about it but most people don’t. This isn’t about when people look at it but for the follow up conversation where you mention “its natural and x carats.”
I'm not suggesting lying about it, I've just never heard anybody show off their finger and say 'it's natural' and it didn't occur to me anyone would. One can just not do that, and as a bonus, seem more normal.
