Patricia Ann Taylor, age 13, of Phoenix, Arizona, for her question:

   How does a cavity get into a geode?

On the outside, a geode may look like a crusty, humdrum lump of stone. But it may feel lighter than similar rocks strewn around, especially if it has a hollow in¬side. If you crack it open, you may find that the inner walls are lined with semi¬precious stones. These may be violet blue crystals of amethyst or petal pink crystals of rose quartz. Some geodes are lined with smooth bands of rainbow colored agates. But the center of the rocky pocket of gems is often empty    and this is mysterious.

The recipe for making a geode began ages ago within a bed of sandstone or some other basic rock. The rock was riddled with empty holes, somewhat like Swiss cheese. Maybe they once were filled with gases that have long since escaped. Rainwater seeped through the bedrock, dissolving a wide assortment of chemicals. Some of it got trapped in a rocky pocket and evaporated, leaving its chemicals behind. Deposits of silicate minerals lined the walls and gradually grew thicker. Given enough ground water and enough time, they would fill the entire pocket. But the slab of basic bedrock at last broke apart, leaving a crust around the gems and maybe an unfilled cavity in the middle.