Ore vein

An ore vein is a rare underground terrain feature that provides massive amounts of ore.

Structure
Ore veins occur as large branching networks of rock with high concentrations of associated ore. Copper veins are largely made of granite associated with copper ore and raw copper. Iron veins are tuff associated with iron ore and raw iron.

Ore veins can snake and branch and eventually dead-end, similar to spaghetti caves in their shape and scale. Where they intersect caves, the player can easily spot them in the floors, walls, and ceilings. In some cases, more than 2,000 of a particular ore can be found in a single vein.

Generation
Ore veins are rare but huge. Copper veins occur above Y=0, and iron veins occur below Y=0. They are overridden by caves and mineshafts.

Technically, ore veins are a part of cave generation and are not decorators. As a result, they can generate in The Void biome.