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The Trial of Stones is a side quest in Hyrule Warriors: Age of Calamity.
The Trial of Stones is a side quest that can be unlocked after completing the Scenario, "Destroy the Yiga Clan!" in Chapter 4.
There will be three quests that can make you qualify for The Trial of the Ancients, and this is one of them. It can be found in the Faron region.
The requirements needed to complete this are 7Ethereal Stones, 10 Ambers, 5 Topaz, 5 Rubies, and 2 Diamonds.
The remaining two needed to qualify for The Trial of Ancients are The Trial of Monsters and The Trial of the Mighty Foe.