Unless you have been locked up in solitary confinement for most of 1980, you are aware that during this year there has been a great proliferation of advertisements for mail order bargains. tThese offers are marked by several similarities.

Most are represented to be "tests" of certain types of advertisements. The bargain is offered as a means of finding out what kinds of ads are effective (i.e.: A rather large ad that asks, "Do you read small ads like this one?"). The item offered is usually made of gold or silver, or includes an emerald or diamond. The price is ridiculously low. There is a strict deadline date (which changes as new ads are published). There is a strict limit of one item per customer (but if you act quickly they'll let you have two). There are also mail solicitations that say you are being offered this wonderful bargain because "your name has been selected" (usually "by a computer"). The vendor's address is frequently but not always a post office box in one small area in the state of New York.

Several inquiries have reached my desk recently regarding a mail solicitation from the "Testing Division" of Abernathy & Closther Ltd. of Westbury, N.Y. The firm's postcard offers to ship "from our vault" a pair of "genuine diamond stud earrings for $5 plus $1 shipping and handling. There is no further monetary requirement."

Anybody who has not already been impressed by all these fancy words is assured that "each diamond of the pair is a genuine .25 pt 10-facet round diamond and will be accompanied by our certificate of authenticity."

To the 14 readers who have asked whether this is really the chance of a lifetime to buy genuine diamond earrings for $6, I can offer only limited advice.

Diamonds are measured in carats. There are 100 points in a carat. The figure stated is ".25 pt." Care should be taken to note the decimal point that precedes the digits. The reference is not to 25 points but to one-fourth of a point, or one four-hundredth of a carat. Diamond dealers tell me that .25 pt chips can be sold at a very handsome profit for $6.

I am not a diamond maven so I don't know whether a .25 pt diamond is readily visible to the naked eye. However, I have formed the opinion that a diamond of this size definitely would not be mistaken for one of the British crown jewels.

I don't know what the earring looks like or what it's worth. I just want you to understand that when a diamond weighs one-fourth of a point it would take 400 of them to equal one carat, and one carat equals 200 milligrams. POSTSCRIPT

While we're on the subject of once-in-a-lifetime bargains, be advised that buzzwords like genuine, vault, monetary requirement and certificate of authenticity have now been reinforced by the implication that the mail solicitation that reaches you is protected under the aegis of one or more law firms.

Mrs. Leslie A. Shifflett of Vienna has forwarded to me a letter sent to her and her husband that carries the heading, "Robert C. Fox and Associates, Attorneys at Law, Counsel to McGovern, Opperman &Paquin, Attorneys at Law, Minneapolis, Minn."

The letter begins, "These Law Offices represent the MCA Awards Clearing House in matters relating to the official notification of National Sweepstakes Winners. You have been selected to participate in the America the Beautiful Sweepstakes. This is your official notice that you have won one of the nine prizes shown on the enclosed BROCHURE. The contest rules are in the brochure and you must meet the simple Eligibility Requirements in order to claim your prize. Arrangements to pick up your prize must be made within twenty-one (21) days of receipt of this NOTICE or your selection will be rendered null and void."

The letter's use of capital letters to emphasize the "legal" nature of its message and its use of legalese (as in "twenty-one (21) days" and "null and void") should be enough to alert the recipient that this is a hype for a familiar kind of sales campaign.

The brochure's small print reveals that, to qualify for a prize, husband and wife must tour a new resort facility with an "authorized vacation director." Do you need to know more?