The largest uncut, rough diamond known to exist in the world today -- a flawless 890-carat "fancy intense" golden-yellow diamond -- will go on exhibit Thanksgiving Day in the gem hall of the Smithsonian's National Museum of Natural History, the museum's director, Richard Fiske, said yesterday.

With skill and good fortune, the rock may yield a single cut-and-polished diamond of 600 carats. This would make it the largest cut diamond in the world, according to the Smithsonian.

The diamond is owned by the Zale Corp. Chairman Donald Zale said the diamond, bought to commemorate the company's 60th (the diamond) anniversary, is not for sale.

On Jan. 6, the stone will be removed for 12 to 18 months while it is studied, cut and polished by a yet unnamed New York diamond cutter. Whether the Zale diamond will be cut in the modern pear shape, the classic oval or the teardrop briolette has not been determined.

If all goes well, the finished stone will be returned to the Natural History Museum for exhibition in 1986.

The current record auction price for a diamond was set Nov. 14 at Christie's in Geneva when a 42.9-carat pear-shaped fancy blue diamond sold for $4.58 million. The previous largest yellow diamond, the 234.5-carat De Beers, was sold privately a few years back, said Albert Freidel of Sotheby's gem and jewelry department, for about $3 million.

Francois Curiel, head of Christie's gem and jewelry department in New York, said he had not seen the Zale diamond. "But a fancy intense yellow of that size is something out of this world. That is unique. Nothing else like it. They could ask whatever they wanted for it."

The price of a diamond, Curiel explained, is based on its color, clarity, cut and carats. A fancy intense is the darkest color of yellow diamond.

A high-quality one-carat diamond costs about $13,000, according to Joe Schussel, publisher of the Diamond Registry. "But you can't multiply by, say, 600 to get a price. Every diamond is different. And the larger the diamond, the larger the per-carat price. A fancy intense yellow would be very rare, like a work of art, a museum piece."

At the moment, the largest cut diamond in the world is the Cullinan I, a white, pear-shaped 530.2-carat diamond, cut from a 3,106-carat rough diamond found in 1905. Two stones from the Cullinan (which yielded 108 cut gems) are now a part of the British Crown Jewels, guarded in the Tower of London.

Zale said, "Fewer than 50 gems weighing more than 500 carats have been unearthed. Even finding a stone that will yield a one-carat gem diamond is one in a million. You might say it defies the odds by its very existence."

In the 1970s, the 968.9-carat Star of Sierra Leone, then the third-largest uncut diamond, was lent to the Smithsonian for two weeks' display. The diamond, which was not flawless, was cut and yielded one 143.2-carat diamond, plus 10 smaller ones.