When it comes to minerals, you certainly can't judge a book by its cover.

Perhaps the best example of that is the geode - a spherical rock that appears quite unremarkable on the outside, but contains a hollow cavity lined with small crystals in a variety of colors.

Geodes are particularly enchanting to children, who will gleefully watch the plain-looking rocks get cracked open to see the sparkling "treasure" hidden inside.

The most prized contain purple amethyst crystals.

At least one geode cracker will be at next month's TCC show, and will probably charge $5 or $10 to break open a rock and give it back to the buyer.

Some may find that their geodes are actually nodules - completely filled with crystals, instead of merely lined with them. Geodes are more valued than nodules, however, so dealers will usually let kids pick another rock if they originally get a less-than-exciting nodule.

Geodes are found throughout the world, but mostly in desert areas - including in Arizona.

For some, encountering geodes at the show might spark a lifelong interest in gems and minerals, said "Bear" Pitts of the Tucson Gem and Mineral Society, which helps sponsor the main show Feb. 14-17 at the Tucson Convention Center.

No two crystals patterns in geodes are ever the same, but colors are fairly limited - either clear, pink or purple.

"It's always a surprise," Pitts said. "That's one of the biggest attraction of geodes. They're always so pretty."

Geodes are also highly prized by mineral enthusiasts, because they're a great way to introduce youngsters to the excitement of rock collecting.

"You're looking at the unknown … which is one of the attractions of earth sciences - not knowing what to expect," said Pitts. "The unexpected is the prize.

"I've met a number of professional geologists whose interest started at a gem show somewhere," he said.

* Contact reporter Bryn Bailer at 573-4119 or at bbailer@azstarnet.com.