Stones of Barenziah

In Skyrim you will find an usual gem then a misc quest is added saying that you have to get it appraised. You can get it appraised by a few people; two of which are Maul in Riften and Vex, part of the theives guild, in Riften. Upon getting it appraised you will find out that there are a total of 24 of them and that they are only worth anything if you have the whole set.