Agate Mountain

Agate Mountain is the mountain in which Darkstalker was trapped underneath for about 2,000 years. It was the tallest mountain in Pyrrhia before an earthquake created a landslide and part of it collapsed, leaving Jade Mountain as the new tallest mountain on the continent. This mountain is in the Claws of the Clouds Mountain range, within sight of Jade Mountain Academy as demonstrated in Escaping Peril and as stated in Darkstalker (Legends).

Trivia

An agate is a cryptocrystalline variety of silica, chiefly chalcedony that consist of a large variety of colours and types.
It is possible that all of the mountains in the Claws of the Clouds range are named after gems.
Agate mountain isn't in the Darkstalker (Legends) map, despite it being the tallest mountain in that time period.