The whole idea of a cap-and-trade scheme is that you fix quantity, not price.

ECONOMIST: Energy

They were commonly used for trade, and could contain information about the origin or quantity of goods.

WSJ: The Ur-Metropolis | Uruk | Pergamon Museum | By A.J. Goldmann

It's calculated 1, 200, 000 tonnes of building materials were pulverised at the World Trade Center and most minerals are present in the dust (not necessarily in a large quantity).

BBC: 9/11 conspiracy theories