Diamonds rarely generate encased in coal

Diamonds will uncommonly generate encased in coal.

Existing diamond veins (above deepslate) have a 7.5% chance to generate coated in coal (by 1 block) so no diamonds will be seen. This mimics real life and is a fun easter egg.

Keep in mind that this will not be very common, so you won't start to see a massive drop in diamond spawn rates. Furthermore, diamonds are relatively rare above Y=0; the majority of them are at the depths of the world, where coal is non-existent. 

Just imagine mining into coal and uncovering diamonds. This would be a rare treat.