This giant mineral-filled cavity is an example of a vug. Such cavities may be formed when bubbles of gas become trapped in cooling magma or when sedimentary rocks are slowly dissolved by groundwater. This example was filled with three different minerals in close association. It takes good timing for a perfect vug to form. If the mineral deposition phase is too short, the cavity will be nearly empty. On the other hand, if the mineral formation phase lasts too long the crystals will grow into one another, losing their individual shapes and turning into a solid nodule. This specimen is part of the Robert R. Wiener Mineral Collection.