It's my opinion they're not an asset, unless you own the cullinan diamond or something very pricey, they're going to depreciate, just at a slower rate perhaps. Older diamond cuts go for less as we get better at cutting techniques, the rest of the ring also loses value as the setting goes out of fashion.

The icky diamond thing is arguable, these are generally mined in countries with decent labour laws. On the contrary, lab diamonds require few workers and takes food out the mouth of poorer mine workers who become jobless.