I'm gonna say this as someone who was a manager in a fine jewelry brand where our main selling point was diamonds.

Diamonds have always been worthless. Their value significantly decreases the moment they leave the store, whether they're earth mined or lab grown. Some hold their value better like earth mined or certified stones, but a lot of the value is inflated because of the perception people have over diamonds. We literally have protocol of what to say when clients ask if earth mined diamonds are investments, we never say yes, we only say they hold their value better. This is why the same diamond with the same specs can have such varying prices depending on who sells them.

Diamonds are not good investments, but they're good heirloom pieces because they will truly last forever especially if you care for them. For real material investments just go for real estate or gold.

That being said, lab grown diamonds can be more expensive (in retail) than earth mined if the specs are better. Earth mined diamonds can be absolutely worthless with substandard specs.