A rock festival that stars real rocks, gems, and minerals

They brought suitcases full of rubies, overnight bags stuffed with diamonds, and attache cases lined with emeralds as they checked into their hotel. This year there were over 200 companies checking in with their crown jewels at the 16 th annual Washington International Gem, Mineral, and Jewelry Show.

What looked like a pasha's ransom glittered from the dozens of booths and tables filling the cavernous, white-columned display halls of the show. Behind one display table a man in a gray suit added up carats as though they were groceries for a woman who had just bought a tiara's worth of jewels. One of the lures of the show, which draws 16,000 visitors in four days in June, is that gems and jewelry are sold for roughly half the price they would fetch retail.

The man who can tell you all about that is gem impresario Herbert Duke, who runs not only this exhibit but more than a dozen others annually from his company, International Gem Shows Inc., in Bethesda, Md.

Mr. Duke shimmered at the opening of the Washington show: On his right hand he wore two large opal rings, which flashed blue fire; on his left hand, a 200 -year-old Italian sardonyx cameo ring, and a large imperial jade ring. His belt buckle, the size of a goose egg, was also opal. ''But it's phony opal, you can see that,'' he said, flashing the buckle back and forth. ''With the real opal you get a change of color; with the phony, the color stays flat.'' Opals were big news at this year's show. One entrepreneur, Dag Johnson, brought with him a million-dollar opal collection.

Mr. Duke, wearing antique cloisonne cuff links with his white-on-white damask shirt with the pagoda design, gray trousers, blue jacket, and striped tie, acted as master of ceremonies. A mustachioed man of medium height with silver hair and a zest for jewels, he is also the publisher of ''The Crown Jewels of America,'' a trade catalog, and leader of gem safaris to India, Nepal, and Sri Lanka.

The ambassador from Sri Lanka, Ernest Corea, in a glen-plaid suit, was on hand to cut the red plastic ribbon on opening day. He told the crowd that Sri Lanka is ''one of the foremost producers of the world's best stones,'' and added: ''Those of you who have been to a gem pit may be surprised that what comes out in a raw and yucky state is what turns into a beautiful stone exhibited here. . . .'' His wife stood by smiling, wearing an amethyst necklace, tiny pearl earrings, and national dress: a purple print sari-wrapped skirt called a redda and a white lacy blouse, known as a hatte.

''Don't ask me to pose again, I just did it for 20 minutes,'' sighs the Sri Lankan ambassador, darting away from the flash bulbs and the jeweled replica of the Crown of Charlemagne. Back in the show's office, Mr. Duke says that the 200 companies here came not only from all across the United States but include Chinese, German, Brazilian, Hong Kong, Sri Lankan, and Australian gem-meisters.

For fans of jewel-heist films like ''Topkapi'' and ''The Hot Rock,'' Duke admits there are occasional moments of intrigue on the glitter circuit. A small gang of jewel thieves zeroed in on some of the dealers during a recent West Coast trip. He says they tried to force exhibitors' cars off the road and slashed tires in parking lots or distracted sellers at their booths to divert attention during theft attempts (the thieves were un-successful).

One of the gang was eventually caught, but the others have eluded the police so far, he says. But Duke emphasized that security guards were out in force at the Washington and other shows, some of them visible but many of them plainsclothes men and women.

At the show, many of the stomes were set out on tables in pans or dishes, like food at a family picnic. At the Prescott Turquoise booth, owner Herbert Kuglmeier displayed his Arizona turquoise in big plastic dishpans, showing the stages from big aqua hunks of stone, through the chemical and heat process, into the sanded and polished pieces used for jewelry.

At Rockyland Imports, the table was loaded with mason jars full of fire opals in water. And at Tom Tanaka's booth you could learn to pan gold in a green plastic tray, where the tiny crusts of real gold suddenly emerged right before your eyes from a heap of dirt washed with water. At the same place, you could have bought a $12.50 bag of sapphire gravel packeged by Gem Mountain Sapphires of Phillipsburg, Mont. With that and an inexpensive panning kit, you can pan your own blue, pink, and yellow sapphires.

Of course, some of the most precious objects were screened by glass: the $100 ,000 rectangular blue topaz that weighed 1,216 carats and looked like a tray full of blue ice, for instance, at the University of Richmond gem museum display. Or the $100,000 Ming dynasty tea service of nephrite jade the color of sea-foam and 600 years old, at the booth of jade master carver Hing Wa Lee. Also behind glass was a sculpture of a boy under a bamboo tree which Lee had carved from 80 pounds of royal blue lapis lazuli into a few inches of treasure priced at $16,000. Out on a table nearby pranced his handsome delft blue cloisonne horses at $4,000 each.

''I want an emerald ring and I don't where to start. Can anyone tell me whether I'm in the neighborhood of $200 or $200,000?'' asked a blond buyer in a white pantsuit. ''It depends on the color, wheather it's clear or not or whether it has inclusions or not, ''said a voice behind the booth at N. V. Malhotra's.

Mr Malhotra displayed a $900 emerald and a $4,000 emerald side by side; the $ 900 one looked like lime Jell-O compared with the $4,000 one, which was a deep, glorious green, Nearby glittered a 10-carat, $12,000 tanzanite of the violet-blue color identified with Elizebeth Taylor's eyes, and a rare green garnet (most garnets are dark red) at $1,500 a carat. At Treasure of the Pirates you could pick up a $12,000 art deco diamond bracelet, 18 1/2 carats' worth of white diamonds, which the Pirates spokeswoman said would retail for $20,000.

Then too, you may never have seen an opalized gastropod, which is simply a snail shell that has taken on an opal crust through centuries of silt from inland seas in Australia. These shimmering snails and opalized plesiosaurus bones were part of the display in Dag Johnson's opal collection, Mr. Johnson is a blond Norwegian who at 16 struck off for South Australia and soon parlayed a $ 4.75 investment in an opal mine into millions. He exhibits a gorgeous hunk of green and blue opal known as the Queen of the Pacific, 30.89 carats' worth of splendor that is, he says, literally priceless; Queen Elizabeth II owns the other half of it.

''A good opal glows in the dark.'' said Johnson, dimming the lights on the Queen of the Pacific, which pulsed with color like the northern lights. The names trip off his tongue: blue jelly opal, precious honey opal, a $49,000 Andamooka opal.

Jean Ryan, of Ryan's Artistry in Gems, Yorba Linda, Calif., says she and her husband ''do 33 shows a year.'' criscrossing the country between trips to Germany and Brazil, meeting many of the same people at the Washington show, which is like a class reunion. The Ryans' piece de resistance is an enhydros with dragons, a piece of lavendr-gray agate carved into a dragon shape, with water trapped naturally between the dragons in the agete. At $10,000, it's the sort of collector's item that will help pay the costs of a trip like this: Travel fare plus $745 for a booth, in addition to hotel room and food bills.