Craziness or brainy investing? You be the judge.

In 1966, a $10 Indian gold piece minted in San Francisco in 1913 sold for $110. In May of last Year, this very same coin, one of 66,000 minted, went for $15,000. This May, it carried an $80,000 price tag . . . and if you bought it for that amount, boy, were you a shrewdie. The going price, would you believe, is now $100,000.

An isolated case, you say. Wrong! The U.S. coin market, at least certain rare coins, is going through the roof. And gains of 400 to 500 percent in just the past year alone are not uncommon.

For example, if in May of '79 you bought an 1883 nickel without the usually accompanying designation of 5 cents, you would have anted up $45. Just a year later, that same coin went for $700 -- a gain of 1,445 percent.

Or let's say you shelled out $2,500 about a year ago for a $3 gold piece. The current price: around $15,000, a 500 percent gain.

All of this raises a legitimate question: Is the U.S. coin market -- which boasts about 10 million U.S. collectors -- overpriced?

Arthur Weiner, a Chicago businessman who heads up a syndicate that owns a number of Panama-Pacific $50 gold pieces (up from $15,000 to $100,000 in just the past 12 months alone), thinks "insanity has reached the coin market."

Declares Weiner: "Too many uninformed people are coming into the market and bidding up certain coins to stratospheric levels -- a frenzied situation that's almost a repeat of the Dutch tulip craze. In trying to beat inflation, people are buying with emotion and without common sense. It's lunacy; they soon could be stuck with illiquid coins whose actual worth bears no relationship to the inflated values."

Strong words, indeed, especially since they come from someone who has every reason to want to see coins go a lot higher in price. Still, Weiner isn't knocking his own $50 gold piece, noting its rarity (only about 300-500 left in the world), its intrinsic value (2 1/2 ounces of gold) and its historical significance (it commemorates the opening of the Panama Canal).

Needless to say, not every U.S. coin is suddenly doubling and tripling in value overnight. But there's no escaping the fact, Weiner's fears notwithstanding, that the public's craving for just about anything that could help temper inflation will likely cause coins to increase even more in price.

Indicative of this is a recent study of assets by Salomon Brothers, the big Wall Street block-trading firm, which shows that U.S. coins are increasing in value at an even faster rate than in the past.

In the past 12 months, for example, U.S. coins appreciated 25.3 percent, vs. a 21.9 percent annual gain over the past five years and a 16 percent yearly advance over the past 10 years.

As coins went, they were the fifth biggest gainer over the past year -- topped only by gold (104 percent), oil (92.4 percent), silver (76.8 percent) and U.S. stamps (43.2 percent).

Norman Stack, a general partner of New York-based Stack's, one of the country's most respected coin dealers (annual sales over $10 million), shares the view that certain coins are ahead of themselves. And one that he thinks is greatly overpriced is the 1913 $10 Indian gold piece mentioned earlier. But he hastens to point out that demand is as strong as ever in the overall coin market.

"As long as high inflation exists -- and I don't see it going away -- I look for rising prices and more and more people turning to coins as a hedge," he says.

Stacks says it's worth noting the stability of the market, pointing to "upward undulation (in coin prices) rather than peaks and valleys."

If the average guy in the street wanted to buy a coin for investment purposes, which one would Stack recommend?

Commemorative half dollars -- struck between 1892 and 1954 -- in mint (unused) condition, he tells me.

These coins, generally ranging in price from about $40 to a few thousand dollars, have been going up at a rate of about 25 percent a year, and Stack expects this trend to continue.