A Keystone (要石 Kaname-ishi) is a holy rock object that can (among other properties) invoke or suppress earthquakes, wrapped in shimenawa. Based on what Eirin Yagokoro said, it is a heavy object made of black granite. Keystones can come in different sizes and shapes.
Only those of the Hinanawi household are able to handle, insert and remove the keystone, and other celestials are not able to use it. As the Hinanawi household handled the keystone before they became celestials, it was not originally something handled by celestials. Although, Iku Nagae and Yukari Yakumo appear to also be aware of the keystone's powers and properties. It is also revealed that Eirin knows the Keystone's functions and the Hinanawi clan to which Tenshi belongs to. Tenshi Hinanawi uses keystones in many ways, including: as danmaku weapons, launched as a drill, as a cushion to sit on, as a bludgeon to attack with, for standing on, and also as a possible counterpart to Reimu Hakurei's Yin-Yang Orb.
Keystones are said to be able to suppress earthquakes by pressing it down into the Earth and preventing the movement of the giant earthquake-causing namazu that live underground; however, the potential for an earthquake builds up over time when it's still inserted in the Earth, but an earthquake will not occur in the place where it is thrust in, but the energy (that bends the Earth) steadily accumulates. In that situation, if the keystone were to be removed, the energy would fully overflow and a great earthquake would occur.
In the air, as the current Heaven, the stones that float there were also originally super-large keystones that were stuck in the Earth. Before Heaven floated in the air, there were absolutely no earthquakes on earth, but when the giant keystone was removed, a massive-scale earthquake occurred, and the living things on Earth were once all swept away, and afterwards earthquakes have begun to occur.
- Scarlet Weather Rhapsody
In the events of Scarlet Weather Rhapsody, Tenshi convinced Reimu to let her incorporate a Keystone into the foundation of the rebuilt Hakurei Shrine, effectively making it a shrine to her clan. This lead an angry Yukari to tear down the shrine again in order to seal it.
- Double Spoiler
- Hopeless Masquerade
Tenshi's cameo in Hopeless Masquerade shows her standing on a keystone.
- Impossible Spell Card
In Impossible Spell Card, being one of the many strong youkai and humans to try and stop Seija Kijin the mischief-making amanojaku, Tenshi again used her keystones as danmaku in her spell cards that are considered impossible to dodge.
Tenshi's pre battle portrait from Antinomy of Common Flowers
Tenshi's keystone in one of the Ability Cards from Unconnected Marketeers