NO TWO COINS are the same, collectors like to say. But how can you tell two seemingly identical coins apart?

Few issues have so inflamed American coin collectors. Since the 1890s, most collectors have agreed the solution should be to grade all coins on a uniform standard. The problem has been that few collectors have agreed on what the standard should be.

Even if they could agree, individual graders often disagree what grade should be assigned to a particular coin. To make matters more complicated, one of the widely used scales provides for 11 separate grades for mint, or uncirculated, coins. Not surprisingly, the same grader has been known to grade the same coin differently on different days.

Now the Professional Coin Grading Service, a Newport Beach, Calif., organization founded by a group of coin dealers four years ago, has come up with what may be the ultimate grader: a computer.

Some coin experts say the idea, two years in the planning, may revolutionize coin grading, forcing many collectors to have their coins regraded with a system that is certain to be more consistent than human graders. PCGS officials are talking much more cautiously, saying that their computer, dubbed "The Expert," initially will be used "in tandem with its human counterparts."

However the system is used, The Expert already has claimed its first victim. Shortly after the PCGS announced it was ready to grade its first coins by computer, the American Numismatic Association, a longtime advocate of grading systems, disclosed it was selling its own human grading service for $1.5 million, plus up to $3 million in royalties.

In its statement, the ANA acknowledged that the future belongs to computer-based grading systems and said it could not afford the costs of setting up a system to match the PCGS's Macintosh IIX-based system.

"We already have spent more than four years looking at computer grading, reviewing a variety of systems to determine how the collector can best be served by computer technology," said ANA Executive Director Robert J. Leuver. "However, ANA was faced with having to spend nearly $1 million to get on line with computer grading and on the {computerized} coin trading networks."

For the moment PCGS is grading only the widely held Morgan silver dollars, at a charge of $26 each, but the service said it hopes to expand it to cover other widely traded American coins, such as the $20 gold St. Gaudens coins, Walking Liberty half dollars, Peace dollars, Mercury dimes and Franklin half dollars.

David L. Ganz, a New York lawyer and recognized coin expert, is one of the computer's fans, describing it as more reliable than human graders. "It's amazing," he said.

The Expert uses a camera system to scan the surface of each coin and scrutinize it section by section for the flaws, nicks and other signs of wear that would detract from its value.

The computer also does what PCGS calls "digital fingerprinting" of each coin, collecting data that can be stored on an optical disk and later used in tracking any coin that might become lost or stolen.

Other coin services have announced that they, too, will offer computerized grading. Amos Press Inc., the Ohio firm that bought the ANA's grading service, said it plans to move quickly into a computer-based operation.

Leuver said the ANA, the nation's largest coin collector organization, was relieved by the sale. It "will allow ANA to independently monitor the grading services and set standards for the industry without any conflicts of interest," he said.

THE SOVIET UNION has announced a bold coin venture, one that attempts to capitalize on Mikhail Gorbachev's popularity in the West and the Soviet's desire for hard cash.

American collectors, however, won't be able to pick up the new gold coins, which celebrate Gorbachev's dual programs of glasnost and perestroika, unless Congress quickly removes a five-year-old ban on importing Soviet gold coins. They were banned from the U.S. in 1985 in the same anti-apartheid legislation that prohibited importation of South African Krugerrands.

Last month the Soviets announced their country "intends to be a major player in the world's numismatic market" and said a new coin it will mint in three denominations will be "an important building block in this plan."

The coins will carry the word "peace" in five languages -- Russian, English, French, German and Japanese -- around a view of the Kremlin's Spasskaya Tower on the obverse. The reverse carries the inscriptions "Mikhail Gorbachev," "Glasnost" and "Perestroika" around a dove carrying an olive branch.

The coins will be .999 per cent gold and will be sold in a one-ounce coin carrying a face value of 200 rubles, a half-ounce coin with a 100 ruble value and a quarter-ounce coin with a value of 50 rubles.

Retail prices will be set by distributors, but Ganz said the coins should hit the markets with a lower markup than many of the other major gold coins on the market. That should make the coins highly competitive, he said.

The Soviets said in an announcement released by Golden Dove Distribution Ltd., the British firm planning to distribute the coins, that they hope to raise $100 million with the sales.

"The coin is a statement to the world of the Soviet commitment to glasnost and perestroika," said Victor Barron, chairman of Golden Dove. "It would be a travesty if this {U.S.} ban was not now lifted . . . to give Americans the opportunity to celebrate in this worldwide message of peace."

Bill McAllister is a member of The Washington Post national staff.