The amber Van Vliet is thought by some to be the king of jars.  I keep hearing different figures on how many there are but there are only three that I've been able to verify.  I think that part of the confusion is that there are different types of Van Vliets - I'm referring to the improved - and there is at least one very deep olive one which I think some people think of as a sort of amber.  In any event, it's very rare and certainly the Van Vliet closure is one of the oddest in the collecting world.

Van Vliets in aqua are very desirable but not that incredibly rare.  In jar terms, this essentially means that there are probably 30-50 of them or so (this is a bit of a guess on my part based on how many collectors I think there are that might buy one and in the belief that most of those collectors actually own one).  In other collecting areas, having 50 of an item would be called uncommonly rare.

The point here is one that I brought up recently on a fruit jar forum - the more common a jar is in it's "common version" the more desirable it is in it's "rare version".  For instance, the amber Imperial jar is, I believe much rarer in both it's "common" aqua version and it's "rare" amber version than the amber Van Vliet, but it wouldn't come close to the price paid for these jars.  Part of that is because of the spectacular closure on the Van Vliet, but part of it is that there are enough aqua Van Vliets to generate a sort of "cult" interest in it.  The Imperial is rare enough in its aqua version that it's much less familiar to the average collector and therefore there aren't nearly so many people wanting to own/see/handle the amber version of it as there are hankering after an amber Van Vliet.  Hence we end up with the counterintuitive situation that the rarity of some jars makes them worth less than other singular examples of much more common jars.