a heavy metal tool resembling a thick rod, historically used for pounding or grinding, often used metaphorically for something requiring great patience to refine.

an idiom describing extreme carnage or a massacre, literally suggesting so much blood flows that it can float heavy pestles

to persevere in a difficult task through constant effort or to study diligently; literally to grind an iron bar down to a fine needle

If one puts in enough effort, even the most difficult task can be accomplished; constant effort brings success.

A ritual object in Buddhism symbolizing a thunderbolt or diamond scepter.