WORTH NOTING ON TV

* TUESDAY

Frontline (PBS, 9-10:30 p.m.): Diamonds may glitter, but they're not as rare as dealers would have you believe, according to ``The Diamond Empire,'' a historical look at the way demand for the gems has been manipulated over the years. This edition of the noted documentary series traces the story of diamonds all the way from the present-day diamond market on New York's West 47th Street back to the discovery of a huge South African diamond mine in 1871.

How the diamond market was cornered - and how it enormously enriched a few families like the Oppenheimers - sheds a new light on those ads suggesting that ``diamonds are forever.'' The program says this romantic notion was concocted by DeBeers Consoli-dated Mines, Ltd. - the company holding a virtual monopoly on the market - in the hope that retail customers will never sell their diamonds. Otherwise, the reasoning goes, a second-hand market might spring up and put a crimp in new-diamond sales.

The show also probes some of the less-than-admirable politics behind the DeBeers mining and marketing operation in South Africa.

* WEDNESDAY

Evening News From Moscow (C-Span, 6-6:30 p.m. EST): A report in Russian with simultaneous English translation.

* THURSDAY

I Spy Returns (CBS, 8-10 p.m.): Long before he was Dr. Huxtable in ``The Cosby Show'' (now in syndication), Bill Cosby was the first black actor to star in a noncomedy series. His vehicle was an espionage series called ``I Spy'' and ran from September 1965 to September 1969 on NBC.

``I Spy Returns'' not only finds Cosby as secret agent Scotty again, but reunites him with the man who played his partner, Kelly (Robert Culp). The retired Scotty returns to action when he decides to follow his daughter and Kelly's son - both involved in an intelligence case - to Vienna. (Incidentally, Cosby is also a crime-solver lured out of retirement in ``The Cosby Mysteries,'' a series of TV movies that began Monday on NBC.)

Please check local listings for these programs.