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Slate, flagstone... these are very generic descriptive words for meta sedimentary
stone that can be split. They can be sandy mud stones intercalated
or just very fine siltstones that split well. ...

These are ink stones.
Often made of slate....

They are slightly abrasive. Mine is about 5k grit (SWAG).

The ink sticks are compressed and glue rich carbon black
and by rubbing the stick on...