'Grain' in the Bible

Another parable set he before them, saying, The kingdom of the heavens is like a grain of mustard seed which a man took and sowed in his field;

And he says to them, Because of your unbelief; for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say to this mountain, Be transported hence there, and it shall transport itself; and nothing shall be impossible to you.