Millstone
A millstone is the heavy round stone used to grind grain, and it shows up in three places with very different meanings. Jesus speaks sternly about anyone who causes one of his little ones who believe to stumble, saying it would be better for that person to have “a millstone” hung about the neck and be drowned in the sea. Matthew 18 and Mark 9 both record it. In Job, the lower millstone is simply the picture of something very hard and firm. The ordinary tool becomes a way to say how seriously God takes the care of the vulnerable.
3 verses
Job
His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.
Matthew
But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.
Mark
And whosoever shall offend one of these little ones that believe in me, it is better for him that a millstone were hanged about his neck, and he were cast into the sea.