James Smith lines his grandsons up by the trunk of his car, hands each a shovel, a screen and a bucket and sprays both kids with insect repellent.

"There's an old mine here with a shaft," he briefs John Steinbock, 11, and Ben Poore, 13. "Stay away from that. There's lots of mica, but the stuff we're looking for is moonstone."

On the grounds of Mica Mine Farm, near Coatesville, Virginia, we head down a hill, through the woods to a stream. We see the tailings of the mica mine, operated by the government during World War II when mica was used as to insulate airplane parts. Unable to resist, we load our pockets with the shiny, multi-layered rocks, but Smith reminds us of the business at hand and we plunge ankle-deep into the stream.

Moonstone, Smith explains, is a feldspar, softer than quartz, which shows a bluish tint or lunar-like glow if you look at it just right. It is swept out of the sandbanks that line the stream, and to find it you have to dig up the sand on the stream-bottom and sift it through a screen.

Ben and John choose a place just downstream of a small waterfall and start digging. With every whitish stone that ends up in the sifter, one of the boys trudges downstream a few yards to ask:

"Is this it, grandpa?"

"It's 'leaverite,'" says Smith of one specimen: "Leave 'er right there."

But after a few more pans of sand the kids hit paydirt.

"There's some moonstone in that chunk," says Smith. "See that flash when I turn it? It's only a little, but it's definitely worth keeping."

Encouraged, the kids move upstream and dig some more.

"Don't walk in the water there," Ben admonishes some little kids. "We're trying to keep it clear."

Meanwhile, John's mother has found some moonstones in the stream bank, and Smith encourages her to stay at that spot.

"When you find a pocket you sit down and you keep working it," he says. Smith himself keeps pushing upstream, disappointed in the size and quality of the moonstones we've gathered and determined to find bigger and better ones.

"I got started in rockhounding after I retired," he says. "My wife saw a book I had checked out of the library on minerals, so she decided I was interested and got me a lapidary machine for my birthday. I joined the Gem, Lapidary and Mineral Society of Montgomery County, and I took some lessons in gem-cutting.

"Our club was founded by some junior-high kis 16 years ago, and anybody over 10 can vote.Rockhounding is one of the few things grandchildren want to do with old guys like me."

Ben and John have learned to recognize moonstone with some confidence, and when we rejoin them their bucket is almost filled. But they're disappointed in the quality of their finds.

"Whatever's really nice is badly fractured," says Ben.

Their grandfather remembers a place on a hill above the stream where he once found some. We trudge through the damp woods to the edge of a ravine and the boys scramble down and up the other side again.

"There's always a bigger and better rock just over the next hill," winks Smith.

A few days later, wearing dry footgear and clean jeans, we gather around a table at the back of the Treasures of the Pirates Lapidary Shop in Bethesda as instructor Bill Capps assesses our finds.

"I've found better moonstones in the driveway at Mica Mine Farm," he says, adding that our stones are too small and too fractured to be cut.

Capps is holding some better specimens, store-bought gems that come from Sri Lanka.

"Is that where we went, grandpa?" asks Ben.

Capps explains that Sri Lanka is Ceylon; that Pope Leo had a moonstone ring; that moonstone is supposed to protect its wearer from cancer; and that if the wearer is born under the sign of Cancer the moon in the stone waxes and wanes. He turns the lights out and we shine flashlights on the stones to see the moons.

When the lights are on, we dab a sticky fluid on the stones -- the better to see the moons -- and go out into the dark parking lot to decide just the right place to cut the stone to show off the moon.

"If you cut it just the right way you can get both a moon and a cat's eye," Capps says.