My father would tell a story of going down to Mexico, where he chanced upon a small rock shop in Chihuahua State. Being a rock hound and having spent years in Colorado - which is a heaven for amateur gem enthusiasts - this was the sort of place my father liked to find. He used to drag me into places like this and spend hours explaining to me all the different rocks and what chemistry they had.
This particular shop in Mexico had what all rock shops have, a bargain bin where ordinary rocks that are mildly interesting make their way ... and this particular bin had a rough yellow lump in it about the size of an egg. My father asked about the price and was told fifty cents. The owner had identified it as an ordinary piece of crystal quartz, and since quartz is ridiculously common in Mexico, of course it wasn't worth anything. My father didn't try to correct the owner; he simply paid the fifty cents and cheerfully made that nice large piece of topaz his own.
To a neophyte, a piece of yellow quartz and a piece of yellow topaz look almost exactly alike. Quartz is a common mineral, cheap, found worldwide, and whereas it polishes up so that its pretty, it can't be cut very well and so it doesn't serve as a precious stone. Topaz, on the other hand, is very precious, and when it's cut produces a remarkable brilliance that makes it highly desirable for jewelry. What's the difference? Cleavage. The quartz is a six-sided crystal. The topaz crystal has eight sides.
I find it remarkable that players seem to think that the value of a given stone or piece of jewelry is completely obvious on sight. In the movies, of course, ALL gems that are found in treasure troves are perfectly cut, huge and plainly valuable ... and there is a clear distinction between the value of such gems. That one there as big as my fist? That's worth a lot more than this one here as big as a robin's egg.
Everything about gems defies that sort of comparison. To begin with, they are very rarely cut. Historically, most gems, especially big gems, were at best polished, and that is how they remained for centuries after being found. Diamonds can't be polished; there's no grit in existence that will wear a diamond down, so for most of history - until diamond-cutting was developed in the 14th century - diamonds were fairly worthless, used as grit to polish other stones. A big, faceted jewel such as those commonly depicted by Hollywood in the caves of Arab Princes wouldn't have existed - a large, rounded, beautiful stone like that in Conan the Barbarian was the norm.
Moreover, a small, brilliant gem could be worth a great deal more than a large dull one. If one had two gems of the same size, knowing which one was more valuable and which was less would have been a difficult proposition for anyone except a trained lapidary ... which party members aren't, generally.
Another point, about polishing. Does it ever strike the reader that the gems list in the DM's Guide includes agate? By and large, agate is a ridiculously common stone, in this day and age the sort of thing rock shops sell to children because it polishes up pretty and shiny. Typically, you can get it for about a dollar a stone, and that's from the fantastic mark-up the shop adds. Slip down to any stony beach anywhere in the world and you can find billions of small pieces of agate as large as the end of your thumb - agate is, after all, just quartz, and the world is virtually made of quartz.
Nowadays, to polish agate yourself you need only a few dollars to get yourself a little drum that powered by electricity, with sandpaper you put in the inside and which 'tumbles' continuously until your piece of agate is nice and smooth. It's the sort of fun hobby that mystifies children, who never imagined something so rough could be turned into something so smooth.
Imagine doing it without the electricity. Imagine a set of employees whose job it is to turn the drum, endlessly, day and night. The value of the agate is not its commonality. It's the amount of effort it takes to make the agate smooth and pretty, so that its translucence catches the light. Moreover, different agates will produce different effects - dendritic agate has patterns that reflect the incongruities existent in the original rock; Cairngorm is grey and ghostly; tiger-eye is bright and featured with small distinctive 'eyes'. These characteristics are visible before polishing only to the trained observer.
I don't tell my players what a particular gem is worth because I don't feel they automatically have that knowledge. This does make it difficult for them to neatly divide up treasure; it means that occasionally someone in the party gets lucky, despite the very best effort of everyone in the party to be fair and balanced. But 'balanced' gaming is bullshit; knowledge is power, and if the characters have no such knowledge, then I am not bound to give it to them.
I conceal a monster's hit points and no one questions the logic of that. I conceal a monster's attacks, their special powers, they bonus toys they may happen to carry, and no one questions the logic of that. Why should a player question the logic of denying them automatic knowledge of the value of things?
Ignorance of the environment is a critical, crucial part of the game's drama. I wouldn't mess with that by slapping a convenient price tag on everything that's found.