You know what would make life so much easier in caves for showing directions on where to go? CHALK!
With chalk, you can leave messages on the walls or on the floor inside of caves or on walls that can be used by players to help navigate or leave little messages (ala Dark Souls) for other peeps. Could also use water from buckets or such to wash it away.
This has a few problems, such as people drawing nsfw stuff, and the implications of making the whole world a draw able surface and loading that all in on addition to the normal world.
It would be great if we could make clearer directions than just crystal stakes, but this would have some problems. Maybe a smaller version of the signs, with simple arrows, that have already been suggested?