Silica gel cat litter, commonly known as crystal litter, is mined from quartz sand.Silica gel crystals are filled with millions of microscopic pores that absorb moisture. Silica (the type of sand found in quartz) turns into silica gel crystals when mixed with oxygen and water. It is produced by mixing silicone with water and oxygen. This creates very small pores within the crystals that are incredibly absorbent—each piece can hold approximately 40 times its weight in liquid.