Crimson Agate is a special resource that can be found in the Dragonspine region of Genshin Impact. It was introduced to the game in update 1.2, and is the special resource that you will need to rank up the Frostbearing Tree.
Much like the Oculi that can be found in Mondstadt and Liyue, Crimson Agate is a limited resource that can only be collected one from each location. You can find all currently know Crimson Agate locations on the mate below.
Crimson Agate will have a deep red glow, making it standout quite easily in the snowy Dragonspine region. You will also be able to see a small indicator for the resource on your minimap when you are close to, similar to the Oculi from other regions.
Remember to bring your Crimson Agate to the Frostbearing Tree regulary so that you can upgrade it and get various rewards, including some that will make your time in Dragonspine much easier, such as the Warming Bottle.