Base AbilitySpell Card's Earth power 15% UP (1 turn).
Max AbilitySpell Card's Earth power 30% UP (1 turn).
Story 1A boulder bound with shimenawa, straw rope decorated with paper used for purification. Unlike Hakurei Shrine's, this keystone is controlled by Tenshi. She can make it move around freely in the air without touching it. She can also make many appear at once with different sizes and shapes.
Story 2The keystone quells earthquakes in the area when inserted into the ground, but it is only a temporary measure. It does not actually eliminate the cause of the earthquake.
Story 3While the keystone is quelling earthquakes, no earthquakes will occur, but the energy from the earth will build up. Remove the keystone, and all that energy will come out at once, resulting in a huge earthquake...
Story 4Other people cannot touch Tenshi's keystones. When Eirin figured this out, she realized that Tenshi was not somebody to underestimate, but she has never praised Tenshi directly. Perhaps she does not want Tenshi to get too conceited.
Story 5If you have keystones and the Sword of Hisou, you can manipulate the sky, the earth, and the people. That should make Tenshi seem dangerous, since she can wield both, but there is no real cause for alarm. The celestial's sheltered upbringing and carefree personality stops her from doing too much harm.