A geode is a hollow in rock that filled with mineral-rich water, allowing crystals to grow inward over a long period. When it is cut and polished across the face, the cavity is exposed as a fan of crystal points — darker purple at the rim, paler toward the centre, because the first crystals to form took the most iron from solution.
The one you receive will differ in size, cavity depth and colour. These are sold by weight and by the quality of the cut face. Roughly 2.4 kg, so it needs a solid shelf rather than a glass one.