In general terms, I have an idea about the major story arc I want my party to take.

They have to retrieve four stones and have them at the right place at the right time for the conjunction of the elements, and so on and so forth blah blah blah epic heroics go here.

They have one stone, but they don't know what it is or what it does. They know it is magical, and has some strange side-effects, but they haven't (I think) quite twigged to the fact that this is the hinge upon which the plot is hanging.

I have set up a situation in which one of their advisors could reasonably have researched it and found out what it actually is. He has encountered the stone already, and wasn't able to tell them anything. Last time they tried to go talk to him, he was out of town (a plot device from a largely unrelated story), on a 'research trip'. So, the information is now available.

They have been introduced to the lower echelons of the BBEG's organization, but also haven't figured out that it is larger than just the one cell they recently took out.

They are now of level to begin the epic quest of finding the other three stones.

What I want to know is, how do avoid the appearance of railroading them into the plot? Basically, how do I spring their destiny on them and make it look like their idea?

I am open to anything. Some ideas I have include having their advisor give them some sort of obscure semi-related poetry fragment / prophecy, or point them to another sage in some off-the-beaten-path location, or even having their mentor-dwarf disappear for asking 'too many questions' and having them go and try to find him, or, the obvious, he just hands them the plot on a platter.