David Sprankle was so excited about his rock yesterday that he couldn't help chewing on the sleeve of his nylon windbreaker.

He cradled the rock in moist fingers, offered it to a stranger, then proclaimed it gold. "It has gold in it, see," said David, a Bethesda 5-year-old.

It certainly sparkled. But like several other people who brought rocks to be identified at the Smithsonian's National Museum of Natural History yesterday, David discovered that his treasure was mostly quartz and pyrite, lovely but common substances.

It was the museum's annual "Identi-Day," a time when an obscure back room called the Naturalist Center becomes a mecca for area rock and mineral buffs with questions about apparently peculiar stones.

Three museum geologists talked to about 250 visitors. They identified iron slag, explained how a walrus tusk became stone, and for the Davids of the group, gently described the differences between pyrite and gold.

At a time of federal cuts and budget squeezes, the Identi-Day continues into its fifth year, largely because it costs so little to hold, said Helene Lisy, assistant manager of the Naturalist Center. Later in the year the center will have other Identi-Days for fossils and American Indian artifacts.

"Unless government people were furloughed," she said. "That would be the only way it would end."

The scene was a cross between a science convention and a grade school show and tell.

One man with eight pens in his shirt pocket wandered among the center's 30,000-object collection, bending over a table now and then to watch a geologist at work. Excited children used cardboard trays to hold their finds, some from gravel driveways, while waiting in line to talk to one of the experts.

Pete Roehrich, an 8-year-old from Annapolis, had an opaque orange stone on his tray. "It looks like an egg," he said. "Hoping it would be a dinosaur egg."

Alas, more quartz, said Leslie Hale, 23, a museum geologist armed with a microscope and an Audubon Field Guide to North American Rocks and Minerals.

James Wright, of Capitol Hill, created a stir when he brought in his walrus tusk. Dense and glossy, it looked as if it might be a part of an art object or a stone Cro-Magnon weapon. Wright said his grandfather, a former United States admiral and explorer, found it in Alaska while on his honeymoon.

"That's really interesting," Lisy said, as she inspected it and as a small group tossed out ideas about what it might be.

"Today, we could say it looks like a fossil walrus tusk, but it really looks like a digging implement," she added.

One group chuckled when Mary Allman, 74, shared a theory about what she found in her Southeast Washington back yard. She had about a dozen chalky round pebbles. Because she found them in August after putting down some horse manure as fertilizer, she speculated that they might be horse kidney stones.

A geologist identified them as pebbles once used to grind wheat in an old mill.

Visitors brought limestone, coral and quartz, lapis lazuli, rodingtite and diopside, 623 specimens in all.

One man had a sphere of quartz that turned out to be a Scottish bowling ball. Another brought a moss-covered hunk of slag from an ancient iron smelter.

Tim Rose, 35, one of the experts and a geology graduate student at the University of Maryland, said the turnout yesterday didn't surprise him. A lot of people collect rocks, he said, because a lot of people are curious.

"Basic curiosity," he said. "It's something that people get hooked on. I was hooked when I was five years old."