Catalyst
A catalyst changes the rate of a reaction without being consumed by it, which means a very small quantity does a very large amount of work — and that a catalyst is usually recovered rather than replaced. Industrial catalysis is where several of the scarcest elements earn their price.
Poisoning is the practical constraint. A catalyst surface that binds a contaminant more strongly than the reactant stops working, permanently, which is why sulfur is removed from feedstocks upstream and why leaded petrol and catalytic converters could not coexist.
Each of these uses is established and cited on its own. Whether something counts as an ore, for instance, depends on grade, price and era — and those qualifications are shown alongside it.