The reddish, porous, brittle coating formed on iron especially when chemically attacked by moist air; by extension, the coating produced on any of various other metals by corrosion. Iron rusts, copper and silver are said to corrode, even gold can be attacked by certain acids or elements. The Hebrew word chel·ʼahʹ, translated “rust” (NW; RS) or “scum” (KJ), is drawn from a word meaning “develop an ailment” (2Ch 16:12); hence with reference to metal, it denotes rust. (Eze 24:6, 11, 12) The Greek word broʹsis means “an eating” (Mt 6:19, 20), while i·osʹ means both “poison” (Ro 3:13; Jas 3:8) and “rust.”​—Jas 5:3.

Ezekiel compared Jerusalem to a widemouthed copper cooking pot “the rust of which is in it.” This rust represented the uncleanness, loose conduct, and bloodshed for which Jerusalem was responsible. The command was given, after cooking flesh in the pot, to “stand it empty upon its coals in order that it may get hot; and its copper must become heated up, and its uncleanness must be liquefied in the midst of it. Let its rust get consumed [and its copper may burn, that its filthiness may be melted in it, its rust consumed; RS].”​—Eze 24:3-12.

Jesus Christ said, in his Sermon on the Mount: “Stop storing up for yourselves treasures upon the earth, where moth and rust [broʹsis] consume, and where thieves break in and steal. Rather, store up for yourselves treasures in heaven, where neither moth nor rust consumes, and where thieves do not break in and steal.” (Mt 6:19, 20) Material wealth hoarded up is put to no beneficial use; idle, it may rust and eventually be of no use even to its owner. In fact, as James warns rich men who trust in material wealth: “Your riches have rotted . . . Your gold and silver are rusted away, and their rust [i·osʹ] will be as a witness against you and will eat your fleshy parts. Something like fire is what you have stored up in the last days. Look! The wages due the workers who harvested your fields but which are held up by you, keep crying out, and the calls for help on the part of the reapers have entered into the ears of Jehovah of armies.” (Jas 5:2-4) Instead of using their riches in the right way, they unrighteously hold them back. The longer this is done, and the greater the corrosion and rust gathered, the greater the witness is against them before the judgment throne of God. The opposite of such failure to use material wealth was recommended by Jesus when he said: “Make friends for yourselves by means of the unrighteous riches, so that, when such fail, they may receive you into the everlasting dwelling places.”​—Lu 16:9.