@DavidSchwartz Scarce is not the most accurate term here, it has several meanings: exists in small amounts, difficult to find, and others — which are not always the same. I think what you want to focus on is — difficult to find/obtain/achieve/complete. So for at least a short period of time, the number of instances of that thing is limited. For example there are endless prime numbers, but they are difficult to find. Numerous Bitcoin blocks with a PoW solution, but for some time only one is found.