Water is clearly important. We literally can't live without it. Yet you can get water in your apartment in New York without paying for it. On modern Roman streets you can wash your hands or bathe your dog at any old fire hydrant. In the office and in the dorm you can drink water from the fountain down the hall.
Diamonds are different. You can live without diamonds. Yet they are very expensive, valued extremely highly per ounce in the marketplace.
Notice how strange this is - how a good such as a diamond, which is beautiful but inessential, could be so much more expensive per ounce than an essential and not necessarily beautiful good, such as water. The case is called "the water/diamond paradox." It is the first and the deepest question of value. What, after all, determines the value of things?