I guess it’s an emotional reservation. It would be a bit like feeling guilty for eating passenger pigeons and contributing to their extinction, especially if you hunted them and left some to rot because it was so cheap and easy to get more. Once they’re extinct you can’t help but look back and wince or blame yourself for being part of the problem.

On the other hand, a skilled lapidary can cut a tanzanite with way more care and attention than some commercial gem cutters that are concerned with stone weight and production volume. A lot of big gems end up as windowed trash, where the center of the gem can be seen through like a glass window. No sparkle or reflections because the gem cutter preserved gem weight to charge more at the cost of beauty, and also making the gem lose even more mass later on if it needs to be recut to have proper angles.

If I screw a natural stone up I would feel horrible though. Synthetics are like “who cares you can buy them by the pound”