Tangible investments--beloved by those who bet against the dollar--have proven Newton's Law. They are dropping like apples on the unprotected heads of those who bought them.

Take stamps--specifically, the red-hot, three-stamp set of Graf Zepplins, which sold a couple of years ago for as much as $12,000. Last February, fine sets went for $5,000. Now they're down to $2,100 if you want to buy, and less if you want to sell.

The law of gravity, however, has not yet been accepted by the well-known stamp company, Scott Publishing Co. Scott's U.S. StampIndex, which is used by many investors to evaluate current market prices, recently showed the Zepps at $5,500. "Our editors didn't see fit to change the price for this stamp," says T. M. Kerrigan, an assistant editor at Scott's Stamp Market Update. "They decided to hold to a fair price, and they see that as $5,500."

So don't judge the value of your stamp collection by the Scott StampIndex alone. Part of that index is mere wishful thinking.

Prices are also down on other stamps that have been especially popular with investors--among them proof sets, foreign stamps and issues in wide enough supply to satisfy a trading market.

Some stamps, however, continue to gain in value--mainly, the limited number of rare issues of interest to serious and wealthy collectors. Some 19th century American classics have been bid to new highs.

Many investors who originally got into stamps in order to beat inflation (or dodge taxes) became fascinated by them and changed into true collectors. As their knowledge and sophistication grows, they add to the upward price pressure on the dwindling number of truly fine stamps available in the world.

Middle-range stamps are down in price, but dealers report that steady demand from hobbyists seems to be keeping them from falling too far.

When investors try to sell their collections, they often find that they inadvertently bought a lot of doctored stamps, even outright fakes. The supply-side theory of stamp investing says that when there are not enough good stamps to go around, someone will supply them.

Investment diamond prices shook out this year. Top-quality stones are down about 50 percent from their highs. Investors who failed to buy from reliable investment diamond dealers or from good jewelers may find that the stones in their collections are not as high a quality as they thought. When you try to sell such stones, you may find their prices marked way down.

The market price for colored gems is impossible to fix precisely. Early this year, wholesale prices for a one-carat, color-4 ruby ranged from $3,500 to $6,700, according to the Collector-Investor Magazine, the spread reflecting what each dealer thinks of his particular stones and the extent of ruby fever in his selling area.

But by any measure, says Sarabeth Koethe, executive director of the United States Gemological Services, Inc., colored gems are down sharply in price.

Buying interest recently shifted to semi-precious stones (now called collectors' stones, to make them sound better.) Prices are said to be up a bit for spinel and blue zoisite. But be warned that the colored-stones market is shot through with fraud. An irradiated topaz or red tourmaline may shine like a star on the day you buy, but fade to colorlessness as the years go by.

Silver and gold coins have followed the metals market down. Rarities still bring good prices, but most coins now sell well below their 1979-80 peaks.

Past experience says that gem, stamp and rare-coin prices will rise again. Investors (especially those who bought at the top of the market) are practically cheering for another outburst of inflation, in hopes of being bailed out of their mistakes.

But if interest rates stay relatively high and the economy low, the next upswing in tangibles could be a long time coming.