A word to describe something that is desired only because it is rare

Solution 1:

That would be collectible

What is a 'Collectible' A collectible is an item that is worth far more than it appears because of its rarity and/or demand. Common categories of collectibles include antiques, toys, coins, comic books and stamps. Items that have been mass-produced, and thus are not rare, are often marketed as collectibles to drive consumer demand.

Read more: Collectible Definition | Investopedia http://www.investopedia.com/terms/c/collectible.asp#ixzz47tKaidlD

Solution 2:

Materialistic comes to mind, but that doesn't exactly mean that they want it because it's rare.

It sounds more like an obsessive collector, but that doesn't quite fit either, because a collector normally collects certain types of items and only those, like ancient artifacts, baseball cards, antique weapons, first edition books, etc.

It depends on if it's any rare item, versus a specific type of rare item.

Hmm... but those describe the person wanting it, not the object itself.

Antique, collector's item, memorabilia, artifact, relic, unique, invaluable, one-of-a-kind, exquisite, incomparable, matchless, peerless, priceless, antiquity, ethereal, commodity.

Hope this helps.