Classic Deepslate - Return of the Ancient Stone From the 2020 Trailer
Remember the grand announcement of the Deep Dark update in 2020! Back then we were promised drastic changes to the game: increased height limit, completely new biomes, fully reworked world generation system and many other cool innovations.
Unfortunately, much of what was planned was never implemented. One of the most interesting examples is the initial concept of underground stone. Originally the developers called it "Grimstone" but decided that this name creates too gloomy an impression.
If you zoom in on the texture of deepslate that we're used to seeing in the game now, you can notice that it was created based on a render from Mojang's wiki.
Our Classic Deepslate resource pack uses that very authentic texture.
Additionally, the resource pack requires texture pack extensions higher than version 7 to work properly
Gallery of Changes

Abandoned cave with applied textures

Ancient city in new appearance

Complete collection of blocks updated with textures