The Rare Gacha contains powerful SS/SS+ cards that can be obtained. However, it costs 4 Dragon Stones per individual roll (of a specified element) or 40 for an 11-roll (goes through all elements). You'll also typically get Fairies or Marrones for each roll, depending on the Gacha present. The standard rates for the Rare Gacha are 20% for an S, and 7% for an SS. However, RNG is not always kind. Generally, there will be specified selections where if you roll an SS, it's guaranteed to be from a select group of cards.