Snapshot 21w08a (released in the eighth week of 2021, hence the name) adds a couple new features, including new variants of existing ores and tweaked cave generation.
The new grimstone block, added in last week's snapshot, has now been renamed to "deepslate", and a new cobbled variant has been added which is obtained from mining deepslate with a pickaxe.
A new variation of ores, deepslate ores, have been added to blend in properly with the deepslate caves. Not all ores have deepslate variants, as not all ores generate this far down in the world.
A new type of ravine has been added, called the crack carver. These generate as narrow, long, deep holes in the ground.
The tuff block has been removed from the casing of the amethyst geode structures and can now be found in amongst deepslate deep underground, replacing andesite.
Textures continue to be tweaked in these snapshots, with all of the regular ores getting a makeover as well as the blackstone block found in the Nether.
World generation also continues to be tweaked, with massive caves making their return, albeit slightly less common than before. Ore distribution has also been changed again.
There is an issue with this snapshot, however: iron is quite rare. A follow-up snapshot is expected within the next day.
For a visual overview of this snapshot, see my video below: