Grindstone is an upgrade material in Arknights.
Description
- 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.
Usage
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.
Acquisition
Store
Up to 15 Grindstones can be purchased at the second tier of the Commendations Store for 40 
each.
Regular drops
| Operation | Sanity | Drop rate | Sanity per drop |
|---|---|---|---|
15 
|
Rare (~31%) | ~48
| |
| 21
|
Rare (~35%) | ~60
| |
| 21
|
Rare (~34%) | ~61
| |
| 18
|
Rare (~25%) | ~72
| |
| 21
|
Rare (~32%) | ~65
| |
| 15
|
Rare | ||
| 18
|
Rare | ||
| 18
|
Uncommon | ||
| 18
|
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 due to it being a special drop there.
Extra drops
Trivia
- 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
- Grindstone Pentahydrate – T4 counterpart
