Anti-Portal Block

It gives you a surface the Portal Gun cannot stick to. On its own that is a curiosity. In multiplayer it is how you build a room nobody can shortcut into, or a jump puzzle where the portals have to go where you meant them to rather than wherever a player points.

The Traveling Merchant sells it for pocket change, so clear his stock every time it appears. He will not be there on the day you decide you need three hundred of them. The dark tile design is lifted straight from Portal, where the same texture means the same thing.