More couples become engaged in the month of December than at any other time of year. That, along with holiday gift-giving and a recent passion for items such as fancy tennis bracelets and pins, makes diamonds an important commodity this time of year.

Unfortunately, consumers are finding these precious stones more costly than they were last year, when $11.8 billion worth of diamond jewelry was bought in this country. The average price of rough diamonds was boosted 5.5 percent earlier this year by the powerful De Beers diamond cartel. Furthermore, the cash-starved Soviet Union has sold billions of dollars in diamonds to De Beers so that it could borrow money from it. That effectively keeps the pricing of Soviet diamonds within the cartel."Anyone shopping for diamonds right now will find their price up significantly because the cost of the rough is up," observed Harold Vogel, a Chicago-based gemologist and member of the International Society of Appraisers. "It may not affect the highest income-level customers, but many people at lower levels cannot afford to buy as a result."

Diamond pricing is a tricky business, and that is precisely why many experts hesitate to call diamonds a true investment.

Consider the wholesale price of a one-carat flawless diamond, which in the past was considered the standard for diamond investment. In 1970 that diamond cost $1,600, but in 1980 escalated to $62,000 because it was viewed as an inflation hedge. By 1981 it had deflated to $45,000, before dropping to $26,000 in 1982 and $13,000 in 1983. After bottoming out at $12,000 in the mid-1980s, the price of that diamond gradually moved up to $14,250 last year and $15,500 this year.

"A diamond should be viewed as an investment in the sense that any time you buy something for $1,000 or more, you certainly have in the back of your mind what you might get if you sold it," said Joseph Schlussel, editor of the New York-based Diamond Registry Bulletin. "On the other hand, it is not an investment in the sense that it is a commodity or stock that you can conceivably sell in a few months for a profit."

The average price for a diamond engagement ring in the United States last year was $1,357. The average size was a half-carat.

While diamonds can be sold relatively easily, it may be necessary to sell at wholesale price if you're in a hurry. Because of the shaky U.S. economy, Schlussel doesn't expect much growth in diamond sales here this year.

"Consumers are much more value-conscious this holiday season, in light of both the economy and the fact that diamond jewelry prices are up 10 to 20 percent at retail, depending on the item," said Jacquelynn O'Boyle, an executive with Whitehall Jewelers in Chicago. "They seem more interested in factors such as the guarantee and the refund policy."

It is always important to find a reputable dealer that you trust. Dealers buy at wholesale prices and sell at a marked-up retail price. Learn about diamonds before you buy. Criteria for diamond quality consist of the "four C's": cut (angles and proportions calculated to produce brilliance); carat weight (a carat equals 100 milligrams); clarity (the fewer flaws, the better); and color (with emphasis upon the "whitest" diamonds with the least color).

"The round stone maintains the most popularity, although any stone chosen really reflects the individual's tastes," said Lloyd Jaffee, chairman of the 3,000-member American Diamond Industry Association, based in New York.

Tastes as to diamond shapes vary with countries, with the marquis shape popular in this country and heart shapes favored in Japan. The radiant, or princess cut, offers more facets than other diamonds. There's a basic emerald cut and also the quadrillion variation, featuring more facets.

"Don't buy a diamond from someone who pressures you, and obtain enough education about diamonds so that you understand them better," advised Schlussel. "I'd ask for a quality grading, but the diamond does not have to have a certificate unless it is $5,000 or more."

After you buy a diamond, you should have your setting checked periodically to be sure the stone is properly in place. To insure your diamond, have it appraised, listing the color, cut and number of flaws.