Grain of Aerosiderite is an important ascension material in Genshin Impact and is used to ascend weapons like Black Tassel, Iron Sting, and Mappa Mare. There are a couple of ways to get it, and one is much faster than the other. The first way to get it is by farming Domains, specifically the Hidden Palace of Lianshan Formula near Mingyun Village in Liyue.
- Domain of Forgery – Trail Grounds of Thunder (Wednesday/Saturday)
- Domain of Forgery: Electrostatic Field (Sunday)
The Domains, Genshin Impacts version of Dungeons, will have rotating rewards, so while you can run them at any time and get something, there are specific days when they will drop the Grain of Aerosiderite. You should always try to plan your ascensions in advance so that you don’t have to wait too long to be able to farm the resources that you need. You will also need Original Resin to be able to claim your Domain rewards, which puts a daily limit on how much farming you can do.
You can also purchase them from the Souvenir Shop in Liyue. Each one will cost 4 Geo Sigils, with a limit of three per day. You can earn Geo Sigils by ranking up with Statues of the Seven in Liyue, and by opening chests in Liyue. You can find the location of the Souvenir Shop marked on the map below.
You will normally need three Grain of Aerosiderite to ascend a weapon, in conjunction with some other materials. Once you have them all gathered up, you can then ascend the weapon and continue to upgrade it that same was as your characters.