Julian Leidman fishes into his pocket for loose change, throwing on the desk a newly minted Susan B. Anthony dollar, scheduled to go into circulation in July.

"There's nothing to this," he says. "It's nonsense. No intrinsic value. Plain-looking. The coins you don't get to see - that's what's exciting."

Leidman has just purchased a 1907 $20 gold piece for about $500,000 - making it, according to the 32-year-old Silver Spring coin dealer, and several of his colleagues in the business, the most expensive coin in the world.

He plans to sell the coin, which he says was the personal favorite of Theodore Roosevelt and the pride of Egypt's King Farouk, for $695,000, whenever somebody decides to pay that much.

"That's only 34,750 times face value," he says casually. "There are lots of pennies selling at a higher rate than that."

The one-ounce, quarter-sized coint that will be delivered by armored vehicle to a Silver Spring bank later this week is a 1907 Indian Head double eagle, the "Rembrandt of U.S. numismatics," according to Coin World. It is the only one of its kind that is, or ever was, in existence.

"This not 'unique' the way people usually use that word - meaning rare," Leidman says. "This IS unique."

Leidman, who says he travels 100,000 miles a year in search of coins, first bought the $20 gold piece six years ago, selling it then for a mere $200,000. He has followed its wanderings ever since.

Although nobody has disputed that Leidman's coin is the most expensive now on the market, Lynn Vosloh, one of the four coin experts at the Smithsonian Institution, says there is no way of knowing which coin is the most valuable in the world on any given day. "You can't really price the coins in our collection, because they aren't for sale," Vosloh said. The market for rare coins fluctuates so much, he said, that almost every day a new coin becomes the highest-priced coin in the world.

"They don't know prices," Leidman retorts. "The highest before this was an ancient Greek coin, sold in 1973, for about $300,00." Leidman adds that a previous owner had insured the Indian Head $20 piece for $1 million and designated it the world's most valuable coin.

The coin's selling price is in part a product of its illustrious history. It was struck during what Leidman calls the golden age of American numismatics, when President Theodore Roosevelt, vowing to bring the aesthethic quality of U.S. coinage up to that of ancient Greece, took a personal hand in designing patterns.

Roosevelt instructed the sculptor and engraver Augustus Saint-Gaudens to create the pattern for the $20 piece, taking the idea from the Indian Head penny. Saint-Gaudens died shortly after creating the coin.

Beautiful as Roosevelt thought it was, the relief of the coin was too high to make mass production practical. So the plan was scrapped, but the coin was not.

Somehow, it found its way into the hands of the chief engraver of the Philadelphia Mint, and shot up in value over the next 70 years as it passed through several hands, including Egypt's King Farouk.

After Farouk was driven from his throne in 1952, the new Egyptian government of Gamel Abdel Nasser sold Farouk's coin for only $3,444, a third of what Farouk paid for it. "They did a lousy cataloguing job," Leidman explains. "And it was one of the worst places to hold an auction. I mean, who could come to Egypt?"

When Franklin Delano Roosevelt became president, he stopped the production of gold coins. And now, the U.S. Mint destroys its "experimental or trial strikes," according to Mint spokesman Eleanor Hayden.

To Leidman, who bought the coin April 21 as part of seven gold pattern coins costing more than $1 million, the golden age is over.

"It's unfortunately gotten to the point," he says wistfully "where there are many, many other things on a president's mind other than coinage. He has to delegate his authority."

Leidman has been collecting coins since the age of 12, and dealing professionally since 17. He does not own his own collection because, he says, "I don't think it would be fair to my clients to cherry-pick."

Sitting in his small office, surrounded by books like "Bank Failures in the District of Columbia in the Twentieth Century," Leidman professes no concern about selling the coin. "It may be a matter of years," he says. "But I'll get to handle this beautiful thing all that time." CAPTION: Picture, Head, top, and tail, above of rare 1907 $20 gold coin.