Collectibles slump, and it may be a good time to buy

Generally, the prices on most ''collectibles'' these days are very low. If you are knowledgeable about an item, now may be a good time to buy. But if you are fairly new to collectibles, here are some tips on markets and buying.

Diamonds: Diamonds are graded in four categories: color, cut, clarity, and caret. If your diamond has been examined by a reputable diamond lab, it will have a certificate measuring your stone by those characteristics.

''Now is a good time to think about buying your average investment diamond,'' says Stanley Urlaub, president of Bentleys Bank Ltd., the parent company of Bentleys Diamond Trust. This kind of diamond (a 1 caret G-VVS1 -- which means a diamond weighing one caret, with a G color, and with very very slight inclusions or flaws) was selling for $3,200 in May 1976. It reached its high in February of 1980 when it sold for $17,500, and is now down to $6,700. Dealers hold that the diamond market has ''bottomed out'' -- that prices won't go much lower.

The Barry Report, put out by certified financial planner Jim Barry, offers tips on buying. Don't buy a diamond sealed in plastic - it doesn't allow for inspection before purchase. Don't rely on money-back guarantees; a dealer may not be able to come through in a down market. Don't buy unless you pay cash. People buying on a layaway plan may find their dealer has skipped town, diamond in hand, before the payments are finished. Find a reputable dealer, one whose integrity is recognized by the industry. (This rule goes for all collectibles.)

Gems: Investing in gems is extremely speculative. It is almost impossible to tell a top-quality gem without expert help - which means the field is wide open to fraud. Only two labs in the United States are considered very proficient in examining gems: United States Gemological Services Inc. in California and American Gemological Laboratories in New York. ''People are taken in by the beauty of gems and become very gullible,'' says the USGA's director, Sarabeth Koethe. The gem market is low now but ''it is fairly up for red spinels, tsavorite (a green garnet), and rare colored sapphires,'' she says.

Stamps: When you are buying stamps, ''stick to the classics,'' says Richard Wolffers, one of the country's top stamp auctioneers. Look for stamps prior to 1930 and not below $200 to $250, advises Joseph Krois Jr. of the National Philatelic Advisors Corporation in New York. And aim for top quality - no faults , no tears, no fading, no missing perforations, and no obscured pictures.

Coins: Donald Kagin Jr., president of the well-established Kagin's Numismatic Investment Corporation, warns: ''Avoid anything issued in the last 20 or 30 years . . . there may be hoards of them in someone's basement.'' Buy top quality , which in a rating of 1 to 70 means somewhere in the upper 60s.

Mr. Kagin agrees the coin market is flat now. ''I've found the market moves in six-year cycles. It should be picking up again in about three months.'' Of course, even in a flat market, the very rare coins always do well. At the moment ''top silver dollars are still very popular, as well as fractional Civil War currency, which is very rare,'' he says. Kagin insists you don't have to spend a lot on a coin to get a good one: ''A top quality coin can still be $50,'' he says. But remember, this price indicates a good supply of a coin.