Rubies

The 3 rubies are needed to form the Imperial Ruby in the vault.

Rubies
The 3 rubies are hidden across the whole map. In the vault, they can be activated to the right of the mysterious wanderer

Locations
The 1st ruby is located south of the Waterworks entrance, behind a pillar to the left. The 2nd Ruby is located to the right of the Satyr boss at the end of the Gardens.The 3rd Ruby is located south of the Corrupted Essence place, between the palm trees, inside a barrel.

To clarify, you'll find this if you go to the Master's Room. Just below there is a singular lightning strike. To the right of that lightning strike is a palm tree. You can go past this palm tree and you'll find the Corrupted Ressurection Stone. Below that stone you can shuffle your way through some palms and find the barrel. If you have trouble finding it on the map, it is directly above the Twin Corridor Key's original position.