Is there an expression describing some thing precious because it's rare?

This is a question similar to A word to describe something that is desired only because it is rare, but I am looking for an expression, a saying, or an idiom use in daily life, not a term or jargon.

I'm also err on the side of natural (not artificial) rareness, and the thing is truly valuable. Something like "diamond is precious because it's rare".

precious few ; precious little*

Its colloquial usage is merely to describe something that there is very little of, but that doesn't mean it hasn't been used in contexts of something actually being precious just through its rarity. Such examples are outlined here:

Sermons, homiletical expositions, and leading thoughts on texts of Scripture

"Although they have been but a few, yet, a precious few."

Life Lessons: Train of Thought

"Enjoy those precious few moments in time you get with your loved ones — that half hour of helping with homework, that hour at the pizza parlor, and even those two hours at the movie theater."

Marnie's Christmas Wish

"Loss and trials change us in ways that we can't imagine are for our own good until after we have gone through the fires of mourning, but then we come out with a greater appreciation for the living amongst us, and the precious few days we get to spend with them."

31 SINS: AN ANTHOLOGY

The feeling was difficult to contain as he neared the conclusion that he was living his last few moments on this planet, inhaling those precious few breaths allotted to him."

...And hundreds of other instances can be found on Google Books:

--Used to describe endangered species:

--Used to describe rare jewels:

--Used to describe rare/unique moments:

*(btw, precious few should be preferred over precious little in this particular usage because little will make the reader misread the object in question as small, and not rare.)

The connotative advantage of few in its relation to precious is that few usually means only a handful, or from any small number above two. For this reason, few connotes a greater rarity than even rare itself does. For example, a rare species could number anywhere from the hundreds to the thousands, but a species that there is few of would only number from the tens to the hundreds, which makes its use an example of litotes, or understatement/underestimate for persuasive effect.

You appear to refer to economic concept of the scarcity principle:

• An economic theory which states that limited supply, combined with high demand, equals a lack of pricing equilibrium. Typically, demand and supply will gravitate prices to a stable balance; however, scarcity of a good or service changes the way buyers will value the purchase, thus leading to new market conditions.