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Grindstone is an upgrade material in Arknights.


A grindstone used to process weapon parts. Can be used for a variety of upgrades and for production in the Factory.

An important tool in component processing. It has stable properties, and will never explode, powderize, or crack.


The first of the Grindstone-type upgrade materials, it is a hardened rock-like material known for its stability which makes it an essential material for the production of weapon and explosive components. Grindstone is also used for Operator advanced skill upgrades and Mastery training, and the Elite 2 promotion of 4★ and 5★ Operators.



Up to 15 Grindstones can be purchased at the second tier of the Commendations Store for 40 Commendation Certificate icon.png each.

Regular drops

Operation Sanity Drop rate Sanity per drop
15 Sanity icon.png Rare (~31%) ~48 Sanity icon.png
21 Sanity icon.png Rare (~35%) ~60 Sanity icon.png
21 Sanity icon.png Rare (~34%) ~61 Sanity icon.png
18 Sanity icon.png Rare (~25%) ~72 Sanity icon.png
21 Sanity icon.png Rare (~32%) ~65 Sanity icon.png
15 Sanity icon.png Rare
18 Sanity icon.png Rare
18 Sanity icon.png Uncommon
18 Sanity icon.png Uncommon

As seen on the table above, the best operation to farm Grindstones is 3-3 if one only needs them, or 4-8 if one needs them to process Grindstone Pentahydrate.png Grindstone Pentahydrate due to it being a special drop there.

Extra drops


  • In our world, the term "grindstone" refers to a sharpening stone used for grinding generally made from sandstone rocks.
  • Grindstones are one of the few T3 upgrade materials that cannot be crafted from T2 materials.

See also