"Square cut or pear shape, thes rocks don't lose their shape," sang diamond maven Lorelei Lee. But they sure can lose their value in a bad slump.

Last January, diamond dealers started talking anziously about whether the sholesale price of investment-grade, one-carat D-flawless diamonds would fall below the "psychologically important" $50,000 barrier.

It did.

Dealers now quote D-flawless diamonds at $25,000 to $30,000 wholesale, which Bernard Cirlin, editor of Precious Stones Newsletter, translates into $36,000 to $37,000 for investors who might want to buy.

If you want to sell, however, you might be offered quite a bit less. On the downside of a market slide, diamonds are notoriously illiquid.

If you invested in the diamond trust offered by the brokerage firm Thomson McKinnon last March, you have lost about half your money. Share values started at $994. By end of October they stood at $565, and that's only an estimated value. hares sell over-the-counter like unlisted stocks, at a negotiated price. I fyou have to sell, you take whatever you canget.

Some investors, eager to take advantage of the current slump, are buying from companies that sell diamonds mainly by telephone. Typically, they tell you that their "wholesale" prices are the lowest in the business. But in fact, they usually charge much more than mainstream dealers. It's the old story: When you buy without knowing anything about diamond quality, clarity, color, cut and market price, you are going to get stung in investment stones.

Telephone-sale companies typically package their stones with certificates that guarantee their quality. But buyers have had some problems with stones that were over-graded -hence over-priced.

Top diamond dealers advise that you never buy a diamond that does not come with a quality certificate from the Gemological Institute of America (580 Fifth Ave., New York, N.Y. 10036), which is the most widely recognized diamond-grading laboratory in the United States. If the name of the lab listed on your certificate is only similar too the GIA's, it is not the real thing. If you previously invested in a diamond without a GIA certificate, send it to the GIA to find out.

The GIA grades only loose diamonds, not diamonds in jewelry settings. Investment stones should be at least one carat in size, for which grading prices start at $98.

Telephone salesmen, incidentally, tell the unwary that diamond prices have never dropped, and they'll send you a chart to prove it. But what they send is a chart of rough-diamond prices maintained by the De Beers monopoly.

It is true that De Beers has never officially dropped its price. But cut and polished diamonds undergo wide price swings. The dealer who points to the De Beers price as "proof" that diamonds are forever, is not a dealer you can trust.

Michael Roman, chairman of Jewelers of America, told my associate, Virginia Wilson, that the price of less-expensive diamonds, used in most jewelry, is not much affected by speculation in ivestment-grade stones. Diamond jewelry usually follows the inflation rate, although the slow economy is keeping current prices pretty level. A number of jewelers are running sales.

The average piece of new jewelry is for fun, not for investment. Except in highly speculative gem and gold markets, you can rarely sell a piece of jewelry for more that you paid.