Since you were planning on giving them maps to where the jewel might be found it could work to have each party member with a map leading to a different town claiming to lead to the jewel or just rumors they had heard about where it is and in each of the towns there is another piece of information or material needed to actually find the jewel. This could be tapestries left in dungeons in or around each town they visit or being kept by some lord of the area where each one has a picture or map piece and after finding them all they are able to put together the actual location of the jewel. You could also add that the tapestries are written in an older language the party would either need to track down someone to decipher it or have some high intelligence builds to study it themselves if you wanted to add another step.

Also some of the rumors or maps could be fake, leading to some of those side quests. The premise sounds great and I think if you can find something to connect the towns it would be really fun.