Queen's Ruby is a special artifact that can be picked up in chapter four (The Western Ghats) of Uncharted: Lost Legacy. It stands out from other secrets, as the Ruby can be utilized as a treasure notificator - the item sends out a short signal each time you're nearby a treasure. Moreover, your controller will vibrate, while Chloe's bracelet will be lit in red. The section below contains more details on how to find the Queen's Ruby.
The Queen's Ruby can be obtained in chapter four (Western Ghans). You have to reach the northern part of the location (shown on pictures above). Use the main path to get there - it will take you to the entrance to small ruins.
After entering, take look at the huge stone map from the picture above. There are eleven statues above it. Check out the mechanism at the bottom of the map - you will learn that eleven Hoysal tokens have to be retrieved and brought here. Their estimated locations will be shown on Chloe's map. You can reach them in any order you want (one of them is in a small room next-door). The detailed description of obtaining the tokens can be found in the part of this guide that concerned with Chapter 4.
After finding all the tokens, go back to the same room and interact with the socle at the bottom of the map - this will transfer the tokens to the stone map. After doing that, the cage with the ruby will be opened, and you will get the achievement called Yas Queen. The game will also unlock passage to the area with monkeys. You can find there three treasures, so think about going there.
As we've already established, the Ruby automatically activates near the treasures (audio signal and vibrations). It can be very useful, but if you don't want to make the game overtly easy, you can deactivate the ruby in the pause menu (see above).
Note - using the artifact doesn't limit the access to any of the game's artifacts, including those that have something to do with finding treasures. If you want to use the ruby, go ahead and don't worry about any side effects.