Why the next unit matters
The paradox disappears when value is measured at the margin: not how important a good is overall, but how much benefit someone gets from one additional unit. In a place with plentiful clean water, losing one cup usually changes little. Losing the water needed to drink would be catastrophic—but that is a different comparison.
Economists call the benefit from one more unit marginal utility. As a person gets more of something, each extra unit often matters less. The first drink after a long thirst can be crucial; another glass when a tap is readily available is far less valuable.
Scarcity connects usefulness to price
Market prices reflect choices made at the margin, but marginal utility alone does not set them. Supply matters too. Water is often inexpensive where it is abundant and can be delivered at relatively low cost. In a drought, or where treatment and transport are difficult, water can become much more costly because the available supply no longer meets demand easily.
Diamonds are scarce relative to the number people want to buy, and bringing them to market involves the costs of finding, extracting, cutting, and distributing them. Their price reflects that limited supply alongside buyers’ willingness to pay for their appearance, symbolism, and other uses—not a claim that diamonds matter more to survival than water does.
The comparison is about one more
The key is to compare the value of an additional cup with the value of an additional diamond under actual conditions. Abundant water can have enormous total importance while an extra cup has little effect on anyone’s well-being. A scarce diamond can command a high price because buyers place a high value on obtaining one more.
That is why a thirsty traveler may gladly pay far more for a bottle of water than its usual price: scarcity has changed the value of the next unit.
