A Weather Stone (天気石 tenkiishi) is a rare type of stone appearing during a change of seasons believed to house an animal inside. It is constantly wet, due to condensation caused by the breath of the animal inside.
Weather Stones in Touhou
Two types are mentioned, Dragon Stones (龍石 ryuuseki) and Fish Stones (魚石 uoishi).
What was thought to be a Dragon Stone was found on the premises of the Hakurei Shrine by Marisa Kirisame and taken home. Discovering it was emitting water constantly, Marisa returns it. Kasen explains that this water is the condensation of the breath of the animal inside. She also says that in Guuisou (寓意草), a man boiled a Weather Stone causing no more moisture to appear, implying the animal inside was dead. So, she says one must not try and break the stone, or else whatever is inside will die. In her story, this was also a Dragon Stone.
Reimu Hakurei builds a small shrine for the stone to attract visitors, only to be told Dragon Stones grow many times in size to accommodate the growing dragon. The stone doesn't grow after all and Kasen says that it's probably a Fish Stone instead. Somewhat thankfully, since upon the dragon's emergence, a great, destructive storm occurs. She says the outer layers of Fish Stones can be scraped away to reveal the beautiful sight of two ageless goldfish playing inside.
Reimu is left with a pair of long-lived pet goldfish in a fishbowl after Marisa breaks the stone trying to file away the outside. Kasen is then revealed to have a raw Fish Stone and that she used to have a Dragon Stone, from which Koutei was born.
- Wild and Horned Hermit chapter 17