It is an alien consciousness (either from the stars, or from another plane) come to earth for an unknown purpose, and it has adopted the PCs as its arms and legs in this world. It is so foreign that it finds it hard to communicate with the PCs, and does so with cryptic sentences, like prophesies. It remains silent for most of the campaign, but every once and a while (when you need a random adventure hook) it pipes up with a cryptic request, like, "Go to the cavern with the blue walls and knock thrice on the green door," which you can turn into an adventure.
You don't ever need to determine why the gem wants them to do these things. Let the players try to puzzle it out. If they come up with something you like, go with it, otherwise it can forever remain the cryptic alien gem that is beyond human understanding.