Pietra serena is known as the stone of Florentine Renaissance, the stone of Brunelleschi, Michelangelo and Vasari. There are many examples of its use, such as the Santo Spirito church or Palazzo Strozzi, though perhaps the bas relief of Donatello’s Annunciation, in Santa Croce, is the most noteworthy instance of the aesthetic possibilities of pietra serena. The Card Castle pavilion is built exclusively with this sandstone, aiming to show and express its beauty and to explore its new applications and possibilities. The project therefore stems from a pure geometric form, the equilateral triangle, chosen for its stability. Three rectangular pieces of the same dimensions, 10 mm-thick stone boards, are assembled together and form the side of the basic triangular unit that structures the pavilion. This stable unit and its repetition allows recreating a massive but at the same time extremely light and subtle new world made out of stone. This way of designing a structure starting from a hollow module generates light and permeable surfaces which convey an image of transparency. Furthermore, the density, homogeneous color and neutral texture give the pavilion an abstract image.