Why ask? Personally I just nerded out over the specs a bit, filtered for colourless and unflawed as far as the naked eye can see (i.e. you can get better but requires a specialist with specialist equipment to tell, so who cares, she's wearing it not collecting or selling it) and then went with the largest I didn't think would look silly (or alternatively that's within budget, whichever limit's tighter).

I don't really think there's an unbiased way to ask. 'Lab-grown or natural' makes the latter sound more 'real' and better. 'Ethical or blood sweat and tears' is obviously out. 'Lab-grown and technically superior for the same money, or natural and more flawed' is about the best I can come up with, but the former is just objectively the correct answer isn't it? I don't see how anyone could understand the question and answer the latter. There just isn't a reason to prefer natural, all else being equal, for an item of personal jewellery that you're going to wear?