Ore vein

An ore vein is a rare terrain feature that consists of a long streak of ore and rock.

Structure
An ore vein consists of large, long, streaking collections of ore and a filler rock material. At and above Y=0, ore veins consist of copper ore mixed with granite. Below Y=0, veins consist of iron ore mixed with tuff. They can also contain raw metal blocks. Ore veins can snake and branch out. They can be overwritten by caves, but the player can more easily spot them in the floors, walls, and ceilings of caves.

Generation
Ore veins are rare and huge, containing a large quantity of ore blocks. In some cases, more than 2,000 of a particular ore can be found in a single vein.

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