One interesting property of both gold and silver is that they do not rust. Rust is a property of iron. The only way gold or silver can rust is if iron is added to the gold or silver. Interestingly enough where gold or silver is used as currency, fraudsters (whether it is the government or individuals) have found a way to cheat the system. If they melt down their silver or gold, they can mix those metals with iron, recast the coins, and double their currency using cheap iron. Several emperors tried this to ill effect.

Just like modern counterfeit money, the ancient counterfeit money was detectable. This they did by weight or water displacement or other means. One sure means was rust; it was proof of an impure coin. Sadly, the coin may have passed to an innocent, unsuspecting individual by the time it was caught. In James, he addresses this:

Jas 5:3 Your gold and silver are corroded, and their corrosion will be a witness against you and will eat your flesh like fire. You have heaped up treasure in the last days.
Jas 5:4 Indeed the wages of the laborers who mowed your fields, which you kept back by fraud, cry out; and the cries of the reapers have reached the ears of the Lord of Sabaoth.

Notice that the gold and silver is rusted which means that iron has been added. Notice that the metals are “witnesses” against them. Notice how dire the rest of the verse sounds. The next verse talks explicitly about what they had done: “kept back wages”. When paying with devalued coins, an employer is defrauding the employee. They had agreed on a price, the worker completed their end of the bargain, and the employer tricked the employee into accepting a devalued payment.

An understanding of economics, history, and metallurgy allows the reader to fully appreciate the illustration in James.