What is the antonym of rubber?

Antonym is something comparable but opposite in it's meaning. Like black and white, ground and sky, big and small. There are many materials that differ from rubber but you cannot completely compare them.

For example, plastic is different matter compared to rubber but it's still not opposite. Nor is stone or glas, it's not an antonym, it's just another material. So I'd say, there is no antonym of rubber.