The Lonely Island
The Lonely Island, as the name implies, is a small island found further away from Dragonspine’s landmass.
Here it is on Genshin Impact‘s map:
The location you see my character on is actually important. You can reach this spot by using the fast travel point near the Electro Hypostasis elite.
From there, just make your way across the ridges and hills.
Eventually, you’ll reach this particular spot which has a Scarlet Quartz node. Destroy it and glide down to the Lonely Island.
Break open the Ancient Rime ice block to reveal a hidden chest. Inside is a secret Crimson Agate.