There are 24 unique (Unusual Stones) scattered throughout the map of Skyrim. If you show one of these stones to Vex (She can be found in Thieves Guild in HQ), she will tell you about other details about the stones.
If you manage to gather all 24 of them and show them to the lady, you can earn yourself the reward.
Following guide will help your cause to collect all the stones but before you start, make sure that you have completed these tasks to unveil the locked areas:
Access to Thalmar Embassy (during the main quest)
Initiate the Dark brotherhood quest series
Ownership of Proudspire Manor in Solitude which requires 25,000 gold
Access to the Arch-Mage’s Quarters which can be achieved by starting “College of Winterhold” quest series.
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