Silver veins/Iron nodes in mountain and swamp are too low to harvest.
The wishbone allows you to discover underground locations of silver and iron in their respective areas in the game as well as treasure. The issue however is that most of the time, these nodes are either; 1# too far underground in the swamp to dig down to because of the way the water level works in the game, or 2# the silver veins in the mountains are too deep to dig down to because of the dig depth limit.
I agree with this issue. The swamp veins are not worth the time due to minimal received materials and always being submerged beneath water.
The danger of digging for it is greatly increased by the water constantly disarming you and getting stuck in a nearly inescapable pit around lethal enemies.
One could argue the risk is worth the reward but only if executed prior to receiving the wishbone or visiting crypts, which ensure a steady supply of iron that negates any need for this risk whatsoever.