What about Irridium? It’s rare on Earth*, as I remember.
*Unless you count the thin layer of it you can find if you dig down to the K-T layer anywhere in the world. It’s right undernearth the thicker layer of ash and bones. 
Ranchoth
Absolutely!. If you ate the Mona Lisa (my personal vote for most valuable Art object), the paint would probably kill you. Or you could put your eye out on the corner of the frame. 
Would something like “The Pentagon” or “Vatican City” count?
If not, how about the Space Shuttle, if you throw in it’s operating costs?
This is the second post in a row I’ve misused it’s. Must be late.
Are we talking about value per quantity or price per item?
http://boards.straightdope.com/sdmb/showthread.php?threadid=76647
The nuclear submarine looks good.
In response to the Mona Lisa post, the painting is NOT insured. At least, that’s what I remember from the Louvre tour guide. It is surrounded by plastic and security and won’t ever leave the museum again.
If General Electric can be considered an item, it goes for over $200 billion, last I heard. Microsoft is up there, too.
As for “what makes it so valuable?”; the answer is always: How much is someone willing to pay? Like art, stock, and real estate, being rare and/or useful doesn’t mean anything if no one will pay the asking price. Economics adjusts “value.”
InstantCrisis
My question is not “what makes it so valuable”, it’s “What makes it an ‘item’?” So many valuable things are made up of components. So many valuable things (like NASA’s shuttles) cost a lot to operate, or to replace.
Maybe a rare element IS the only correct answer, because any other item is essentially made of lesser, cheaper itemS. Then again, perhaps the design of the Space Shuttle (or some other engineering marvel) is the really valuable ‘item’, that cannot so easily be pieced together.
If such a thing actually existed, probably the most valuable object possible would be the only dose of immortality serum.