Fred Thompson, age 12, of Des Moines, Iowa, for his question:

HOW ARE GEODES FORMED?

A rock collector regards a geode as a special treasure. When found in its natural state it is disguised as an ordinary pebble, though rather lighter in weight. When we sliced one in half, we see that the inside is a pocket, filled or more often partly filled with glassy crystals. Ages ago, it started out as an empty pocket in a solid bed of limestone or some other rock. The semiprecious jewels were added, molecule by molecule, through ages of time.

Patiently the rainwater seeped and percolated through the original bedrock, dissolving particles of silica and perhaps other minerals. Moisture was trapped in the rocky pocket long enough to deposit its dissolved chemicals. Gradually, they lined the pocket with crystals. The secret treasure may be glassy clear quartz crystal. Sometimes fragments of other dissolved minerals colored the crystals with flowery pinks or purples.