Starving plate is built on the principles of my A Blessing and a Curse document, but is balanced to be useful to some, but detrimental to most.
Armor (plate), very rare
Devour Metal. Any non-magical weapon made of metal that hits the wearer or would hit an AC of 10 is partially absorbed. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls on a hit or a permanent and cumulative -2 penalty to damage rolls on a miss above 10. If its penalty drops to -5 or below, the weapon is destroyed. Non-magical ammunition made of metal that rolls above 10 on attacks against the wearer is destroyed after dealing damage if the wearer was hit.
Bare metal touching starving plate or held by the wearer is devoured at a rate of 2 pounds per second and weapons suffer the same permanent and cumulative -2 penalty as when devoured on a failed attack that strikes the armor.
Each time the starving plate devours metal, increase its weight by 1 or 2 pounds for a hit or miss above 10, respectively. For each 10 lb. above the native 65 lb., increase the AC and strength requirement of the starving plate by 1. When the starving plate is doffed, this metal is excreted as lumps of refined ore and the starving plate returns to its original weight, AC, and strength requirement.
Once donned, the starving plate cannot be doffed unless in hibernation. Starving plate begins hibernation minutes after it devours heavy metals such as platinum, gold, mercury, and lead. A pound of such material causes the starving plate to hibernate for 10 minutes, thus 25 gold coins is usually enough to cause a hibernation long enough to doff the starving plate. Material that causes hibernation is not excreted when the starving plate is doffed nor does it increase the weight of the starving plate, it seems to be consumed without any waste. While hibernating, starving plate does not devour metal of any sort.
If you liked this puzzle of a magical item, you might like the Well of Lost Reflections.