A.T. Robertson Commentary James 5:3

"Your gold and your silver are rusted; and their rust shall be for a testimony against you, and shall eat your flesh as fire. Ye have laid up your treasure in the last days." — James 5:3 (ASV)

Are rusted (κατιωτα). Perfect passive indicative (singular for χρυσος and αργυρος are grouped as one) of κατιοω, late verb (from ιος, rust) with perfective sense of κατα, to rust through (down to the bottom), found only here, Sir. 12:11, Epictetus (Diss. 4, 6, 14).

Rust (ιος). Poison in James 3:8; Romans 3:13 (only N.T. examples of old word). Silver does corrode and gold will tarnish. Dioscorides (V.91) tells about gold being rusted by chemicals. Modern chemists can even transmute metals as the alchemists claimed.

For a testimony (εις μαρτυριον). Common idiom as in Mt 8:4 (use of εις with accusative in predicate).

Against you (υμιν). Dative of disadvantage as in Mr 6:11 (εις μαρτυριον αυτοις) where in the parallel passage (Luke 9:5) we have εις μαρτυριον επ' αυτους. "To you" will make sense, as in Mt 8:4; 10:18, but "against" is the idea here as in Lu 21:13.

Shall eat (φαγετα). Future middle (late form from εφαγον) of defective verb εσθιω, to eat.

Your flesh (τας σαρκας). The plural is used for the fleshy parts of the body like pieces of flesh (Revelation 17:16; 19:18,21). Rust eats like a canker, like cancer in the body.

As fire (ως πυρ). Editors differ here whether to connect this phrase with φαγετα, just before (as Mayor), for fire eats up more rapidly than rust, or with the following, as Westcott and Hort and Ropes, that is the eternal fire of Gehenna which awaits them (Matthew 25:41). This interpretation makes a more vivid picture for εθησαυρισατε (ye have laid up, first aorist active indicative of θησαυριζω, Matthew 6:19 and see Pr 16:27), but it is more natural to take it with φαγετα.