Rubies and garnets, 50 cents a spoonful; amethyst geodes, the size of a fireplace log, their insides a fantasy cavern of purple gem stalactites; seed pearls, little larger than a cumin seed, strung together in a necklace; diamond rings and chunks of cut emeralds worth a sultan's tribute; turquoise by the pound; tiny cloisonne' beads; dresses inset with semi-precious stones and embroidered with gold threads, even pyrite, proving again that all that glitters isn't necessarily gold . . .

The Persian poet Omar Khayya'm wondered "what the vintners buy half so sweet as the stuff they sell." You could wonder the same about the Ali Baba Cave of treasures on display through Sunday at the Sheraton Washington Hotel.

The fabled jewels of the world are cupped on saucers, piled on tables, and stacked in cabinets for the greedy eyes of the onlookers at the 16th annual Washington International Gem, Mineral and Jewelry Show, which opened yesterday. The show is open from noon to 8 today; 10 to 8 Saturday and 10 to 6 Sunday.

About 200 prospectors, lapidaries, jewelers and sellers from China, India, Hong Kong, Texas, Mexico and other states and countries are showing their wares. People who think that gems begin with diamonds and end with pearls will be surprised to find how many colors and shapes stones come in.

"When they come out of the gem pits, they look so . . . yuck," said Sri Lankan Ambassador Ernest Corea. "And then, by the wonders of craftsmanship and even technology, they turn into these wonders."

Corea cut the ribbon for the opening yesterday. "Sapphires, especially blue sapphires, which are rare in the rest of the world, are Sri Lanka's second biggest source of foreign exchange, just after tea," he explained. "We also mine cat's-eyes, topaz, and other stones."

Herbert Duke, president of the show, showed Corea the bejeweled and silver elephants that he bought in Sri Lanka.

"These are very important in my country," said Corea, "because they are reminders of the great Kandy festival with its processional of decorated elephants. You see this silver container riding on the elephant's back--that is a miniature of the holder for the sacred relic of the tooth of Buddha."

Many of the vendors started as hobbyists, and still keep their enthusiasm.

Dale and Hazel Bierer, for instance. "He had this hobby of carving pigs from stone," said Hazel Bierer. "He took them to a show and sold them out in two hours. We were hooked." The Bierers not only sell precious stones, but also the laser-made marvels that sell for as little as 25 cents a carat.

"It takes seven steps to make those cloisonne' beads," said Riley Ryan, who imports them from China. "The brass wire is inserted so carefully, you can run your fingers over it and never tell it's there."

Victor Lu, who has lived in Hong Kong, Africa and other far places, sells amazing chains of shells as well as minerals.

Kim Gandhi of Bombay made dresses out of saris done in intricate embroidery with gold and silver, sequins, stones and glitters from the Mogul period.

If you've always wanted to run your hands through rubies, the show is the place to go. But don't let the gems stick to your fingers. Guards are everywhere.