system
Given the following post, respond as if you're commenting on Reddit:
human
[Portal] What makes a surface portal conductive/non-portal conductive?
gpt
The surface must be reasonably free from vibration, and it must be _reasonably_ flat. Pits and crevices in the surface up to 2 cm deep and 1 in wide seem to be fine, though protrusions must be paper-thin in order for the portal to "cast" over them.

The surface material itself must have specific conductive properties. Ideally, stable, isolated ferromagnetic particles must be present but INSULATED from one annother. Concrete, or specially treated moon dust in gel, has sufficient suspended iron to do so perfectly. A damp rock, a solid iron surface, and plywood, for instance, doesn't.

This is a major reason why portal guns were never sold to the US military; Aperture felt that the device was unsellable because too many structures are made of wood.