There is a jewellery shop in Gerudo town that offers a range of necklaces that provide some useful abilities. When you get to the town, the woman working here will tell you that the owner went off to get some supplies at the Toruma Ruins to the west and has not returned. This will trigger the Missing Owner side quest.
You will need to head out to the Toruma Ruins which can be found on the western edge of the Gerudo desert. Before you go, you need to be prepared for a fairly difficult boss battle against a Moduga, which is quite difficult. Be prepared for it to take you out in a single hit!
Finding The Missing Owner
The map below will show you exactly where to find her. She will be standing on a stone pillar and you should be able to find her location from a distance away as she has a fire lit next to her and the smoke can be seen from a distance. This should help guide you to where the missing shop owner is.
Killing The Moduga
You may have found the missing shop owner but she can’t return to her store until you kill the Moduga that is circling her in the desert sand. This is not an easy boss fight due to the Moduga spending a lot of time under the sand and when it does surface, it can take several hearts in a single hit, potentially killing you in a single strike.
Once you have killed the Moduga, you can speak with the shop owner and she will thank you and tell you to come see her back at Gerudo town. Return to the town after and she will give you a diamond. A very useful item to have if you are looking to get lots of Rupees as these diamonds can be sold for 500 each!