Sibilian Crystals are found only on Big Worlds with a chlorine atmosphere. In form they are a yellow-green crystal with five sides. The valuable ones are always of similar size, shape, and coloration. While some crystals are used for jewelry, the primary consumers are Psionic Institutes. Psionic users have a way tuning themselves to the crystals and apparently being able to boost their powers.
Because of their psionics connections many worlds ban the importation of the crystals.