Mineral crystals grow in empty spaces in rocks called vugs if they are large and geodes if they are small. In the case of geodes, the crystal filled space is surrounded by a rock matrix and sometimes the rock matrix is in the form of a nodule. This collection of quartz crystals is about six feet across and came from a huge vug in a mine. Many valuable crystal vugs have probably been lost when the crystals were mined along with the ore. Occasionally, a collection of crystals is preserved.