What World Coins Are So Rare, That Even Finding Photos Of That Particular Coin Are Hard/Impossible

Baktria 20 drachms. Unique.
Was in the British Museum, but it was stolen, never recovered. It was photographed before it was stolen, but cannot be photographed now.

It should also perhaps be added that just because you can't find a picture of it, doesn't mean it's rare.

Saw a train wreck of a thread on another coin forum, where someone posted a 1985 Bermuda cent, and because they couldn't find any other pictures of 1985 Bermuda cents anywhere else on the Internet, they assumed and insisted it must be super-rare and valuable. Nope, it's just such a common coin, and found across a fairly broad date range, that nobody had ever bothered photographing one of that specific date.

While this scenario is less likely for modern coins, it's really rather common for older coins that have extensive date ranges.

A rare coin, no matter how rare it is, is highly likely to get itself photographed and splashed across the internet, if it (a) gets sold by a big auction house, or (b) becomes part of a prominent Museum collection such as the Smithsonian or British Museum. A rare coin that stays in private hands and/or is locked away in a small obscure museum that isn't disseminating its collection catalogue, is not only unlikely to be photographed, but may not even be properly catalogued - so collectors at large will likely be completely unaware of its existence.

Most mysterious of all, of course, are the rare coins that are photographed and catalogued in old auction lists or otherwise have records of existence, but which seem to have disappeared completely. Like that Bactrian coin Sel mentioned. Or 1964-D Peace dollars.

Thought about this topic in my area of interest - Spanish colonial. I've heard the Chiloe peso is rare enough that only one genuine example is known and others sold at auction are counterfeits or forgeries. You can find pictures of it online, but it's unclear if they are pictures of the genuine coin or the fakes. Also, Carlos Jara's book on the subject only had 40 copies printed... so I'm hoping to snag a copy sometime in the next decade to learn more, lol.

Many Iranian Civic coppers are quite rare.
They were ment for local usage only and generally for a limited time. Those that survived are often well worn.
On this one, I suppose the mint any maybe year of issue to be onthe center cartouche between the scales.
I did not manage to find another picture of this coin on the net.

So should you happen to find a picture of this coin with a cartouche that is not worn down, pleqse don't forget to let me know.