Are diamonds a good investment?
The fairest answer I think we can give here is that it's very rare that a retail product increases in value over time and you can realize this value as a consumer. Rare and iconic pieces such as certain models of cars, watches and other luxury brand items can and do increase in value over time. The same has been true about natural diamonds over the past several decades and more. The challenge is actually being able to sell your item so that after marketing costs, you find an 'end consumer' and get the full price it's worth. Unfortunately, just to get discovered, this often requires a cut of the sale and some kind of discounting.
When selling back to the industry, to a jeweler or diamond trader, you will need to discount your item, as raw materials, such that it's less than the price they have available to them on the wholesale market.
Diamonds are, however, a very good investment as an accessory that can be enjoyed for lifetimes and generations without any loss of aesthetic or intrinsic value. A timeless design, slow fashion item such as this is a far better investment than almost any other item of clothing or accessory.