St. George, the original knight in shining armor, came to the Museum of Natural History yesterday, and there he sits now, nearly 7 inches tall, in a glass case across the room from the Hope Diamond.

The statue, crafted in Russia in 1882, possibly for Czar Alexander III, was given to the Smithsonian yesterday "as a Christmas present to the American people" by John Levey, a retired inventor and manufacturer from Los Angeles. Last July, the official appraisers of the Soviet Ministry of Culture estimated its value at 1 million rubles - $1.5 million at the official exchange rate.

Washington suffers no shortage of equestrian statues, and most of them are more heroic than the latest arrival, at least in size. But George is attracting special attention in the Hall of Gems-partly because his intricately engraved suit of armor is made out of 18-carat gold.

With him is his constant companion, the dragon. This one is rather Oriental-looking, with multicolored wings, emerald eyes and rubies set like warts into the tail that is coiling around the right foreleg of George's horse. A tongue of flame (red enameled gold) is corkscrewing out of its mouth as it snaps at George's lance - a tough mouthful, probably, since it is liberally studded with diamonds.

In all, there are more than 340 diamonds set into the sculpture, as well as 17 cabochon rubies, opals, emeralds and a malachite base. The saddle and some other details are made of platinum.

An air of mystery surrounds the statue, which Levey says he has owned "for four of five years." He said that he does not know how or when it was brought out of Russia or who was its previous owner.

"It's not the sort of thing you bring in a handbag or the sort of thing you smuggle through customs," he speculated. "It must have been brought in either with the full knowledge of customs or under the protection of diplomatic immunity. As to where it came from, all I can imagine is that at the time of the revolution a lot of people acquired things that weren't necessarily their own. The dealer who sold it to me said that it belonged to someone whose name was in the press daily and had been for years, but that's all I know about it.

"When I asked about its origin and history, the dealer simply told me, 'The piece has to speak for itself, Mr. Levey.' He also told me it would 'blow my mind,' and that's just what it did."

Both Levey and Paul Desautels, the Smithsonian's curator of gems who persuaded him to make the gift to the museum, were enthusiastic about the statue's workmanship. "I hope the museum sets up some kind of magnifying glass so that people can examine the fine details," said Levey. "For example, there are picutres engraved on the knee-plates of the armor, and tiny screw heads at the points in the armor plate where screws would be in real armor."

Desautels, pointing out such fine details as the small gold nail heads in the gold horse shoes, compared the statue favorably to the better-known work of Faberger, a contemporary and rival of the statue's maker, master silversmith G.P. Grachev. "Faberge had a good public relations agency, I guess," Desautels said.

Almost the only thing missing in the elaborate detail of the St. George statue, in fact, is St. George himself. "The visor on the helmet is hinged and you can lift it up," Desautels observed. "When I first opened it, I expected to see a little face inside, looking out, but there's nobody home-just an empty suit of armor."

This absence accurately reflects the status of the subject, St. George, an early Christian martyr whose story is rich in legend but rather scanty in factual details. He is mentioned in a fifth-century liturgical text in a list of those "whose names are justly reverenced among men, but whose acts are known only to God."

The name of George (which means "farmer" in Greek) first became popularly associated with the story of the dragon killer in the "Golden Legend" of Jacobus a Voragine, a medieval best seller compiled late in the sixth century. The "New Catholic Encyclopedia," which tends to take a dim view of saintly folklore, suggests that George inherited his dragon from Greek mythology.

In his heyday, George was enormously popular, particularly in the Middle East where he apparently spent his life and in England, where he was named patron saint during the reign of King Edward III. His popularity in Russia is reflected in the fact that one of the Socialist Republics, Georgia, bears his name.

The subject of George and the dragon was one of the most frequently used in medieval art and continued to attract some artists through the late 19th century.