Albert Barnes Commentary Numbers 31:50

"And we have brought Jehovah`s oblation, what every man hath gotten, of jewels of gold, ankle-chains, and bracelets, signet-rings, ear-rings, and armlets, to make atonement for our souls before Jehovah." — Numbers 31:50 (ASV)

The “chains” were “armlets” (2 Samuel 1:10). The “rings” were “finger-rings,” or “seal-rings”; and the “tablets” were worn suspended from the neck (Exodus 35:22).

To make an atonement for our souls before the Lord – Compare Exodus 30:11-16. The atonement was not for any special offence committed (which would have called for a sacrifice of blood-shedding), but rather like the half-shekel given at the census in the Book of Exodus (in the passage cited), was an acknowledgment of having received undeserved mercies. These, if unacknowledged, would have entailed guilt on the soul.