AbdulAlhazred
Legend
Seems odd to me, like you've authored a story and the players are just going to supply some color almost. I mean, you seem to have an agenda for this story of the 4 crystals to happen. It doesn't even involve the players at all (the PCs are part of your plot, but since the players don't even know the story exists they're not even along for the ride at this point!). In terms of contrivance though, which is more to the OP's point, how does this not qualify as a tremendous contrivance? Of all the possible characters who could dig these things up, its the PCs!Fair enough.
With the sort of games I play in and run, I've learned the hard way not to tie prophecies or great events or suchlike to any one particular character for two reasons.
One - less common - is that other players might feel resentful that the one character/player is being singled out for special treatment.
Two - much more common - is that the key character will inevitably find a way to permanently die at the first available opportunity.
And now I'm learning not to even tie them to entire parties! See spoiler for details.Just recently I've been running what's supposed to be a mini-AP baked into my larger campaign, involving the finding and reuniting of four Prophecy Crystals and then maybe having that party use them for something really big down the road.
When another DM ran a similar series many years ago, the second crystal they found got shattered by a lightning bolt; this almost destroyed the whole story arc. Lesson learned - I made these ones pretty much indestructible. The Bag of Holding my lot put their first Crystal into, however, didn't have such protection...and sure enough a few sessions later up it went* with all contents lost. I maintained my stoic DM face at the table but inside - as you might imagine - there was a pretty big facepalm.
This was just a couple of sessions ago. They don't even realize quite what they've lost yet, as they don't know anything about the significance of these Crystals or even what they are; all they know so far is that some adventurers were investigating them several decades ago. As DM I've no idea how they're ever going to get the lost one back or even if they can; but at least they've now found the second one.
* - as fate would have it, again by lightning.
Now, I have an explanation for this, in say DW terms, the PCs are the big damned heroes! They are THE Wizard, THE Fighter, THE Cleric, and THE Thief. Maybe they will or won't dig up the fabled 4 crystals, but their players probably decided that was a thing to begin with! (I mean, the GM in DW could generate this story element as a part of a Front, or maybe even as a soft move/doom but IMHO that would generally only be a good move if something the players had decided at some point thematically signaled that as a story direction that would be in keeping with the course of play).
So, for instance, I might invent something like that if maybe the thief was gunning for a big treasure and bit when offered a fantastic gemstone. Later this gemstone leads things into a whole complex plot surrounding the necessity to get 3 more and avert the final evolution of a campaign front (the exact nature of which will certainly riff off of the choices of PC background, answers to questions asked of players, etc.). Note that DW Fronts "just are" once they are established, if the players send their characters off in some other direction, well the front does its thing and so be it. If some of them die, or even if the party gets wiped and there's a restart, things can go on, no one PC need be cast as THE FATED ONE, although it may turn out to be so when the time comes!
In my first 4e campaign, after it matured a bit past the early set piece stuff, one of the PCs became a sort of 'fated champion' character. The player had given her a background that included having a prophesy, and an unusual upbringing where her family origins were unknown. The monks who raised her CALLED her the 'Chosen One' (or something like that, I forget exactly) but it was only when the PCs got to Paragon that this really started to play out seriously. It was never a given that it would, although clearly since the player created those background elements it would have been kind of a let down not to go forward with it.
IIRC a few times at lower levels the players remarked that some bad guys seemed to want to take out the cleric (this character) pretty badly, but it could be attributed to smart tactics. Finally around 8th level the party got themselves wrapped up in some weird Eladrin (Eldar) politics due to one of the PCs being a runaway Eldar Princess. So they ended up looting an ancient Eldar fortress and found an artifact that triggered off the rest of that story arc (being the savior of the land, basically). Now, if said PC had died, or the player had just ignored the plot hooks that would be fine too. The players could have invented a bunch of different paths to go down, I'm sure. I only invented the Shield of Kinnis plot arc/artifact because it matched up with some lore and fit the character conception really well.