It might surprise you, but diamonds are extremely common, so common in fact that they ought to be one of the least valuable gems instead of one of the most. In terms of sustainability, diamond mining operations often exploit workers for little to no pay and operate with little regard to the surrounding environment.
Besides that, though, diamonds are commonplace in our society. You can't walk through a store without coming across someone wearing a piece of diamond jewelry.
Frankly, if you want a unique ring, avoid choosing this nearly-see-through gem over something vibrant and iridescent like opal.
If you are choosing a birthstone ring and your partner was born in April, though, you get a pass.