CHAMPAIGN, Ill. (AP) _ At Maarten de Witte's school, cutting class is the only class. And the subject is the very hardest - diamonds.

With the American School of Diamond Cutting, de Witte wants to break open the craft passed from father and son, or done in assembly-line fashion to prevent workers from learning all the steps.

''We need to get rid of the secrecy and centralization in this trade,'' he said.

De Witte, a 1975 graduate of the school, took it over from his mentor, Dutch immigrant Leonard Ludel, and moved it from Nevada to Illinois earlier this year.

Though diamonds come mostly from Africa and the Soviet Union, most cutters are based in New York, Tel Aviv and Antwerp. Ludel opened the school in 1967 in Gardnerville, Nev., to give more people a chance to learn the trade.

''Blacks in Africa where the diamonds came from never had the opportunity to learn to cut them,'' said Ludel, 78. ''And there were a lot of Americans who wanted to get into cutting but had no place to study.''

De Witte has been working with two students since moving the school to Champaign and expects the formal opening around the first of the year when he receives state accreditation.

He plans a total enrollment of about 18 for three, 30-week sessions. Tuition is $4,500 per session. De Witte said his school is one of only two in the United States devoted to diamond cutting.

''This is really nose-to-the-grindstone work, and what I look for are people with a strong desire to do it,'' said de Witte. ''If they think diamonds will make them a ton of money and they won't work hard, I discourage them.''

De Witte, who grew up here and later worked as a diamond cutter in Phoenix and New Orleans, said diamond cutters can earn $40,000 a year.

But ''the sky's the limit'' for someone who can grade and appraise stones, and buy old diamonds and recut and improve them, he said.

''The largest stockpile of diamonds in the United States is on women's fingers, and most of them are poorly cut,'' said de Witte. ''We teach students to do a very precision cut.''

De Witte teaches his students every phase of the trade.

In his classroom, equipped with about $15,000 worth of tools, de Witte places a tiny diamond in an old-world tool that holds it. He then leans in to a grinding wheel that resembles a fast-spinning record turntable.

The coating of diamond dust on the wheel begins to shave the stone, creating the desired angles. A diamond is so hard only diamond dust will cut it.

De Witte lifts the diamond and inspects his work through a magnifying glass, then resumes cutting.

It can take weeks, even months, but when the cutting is done correctly, the stone will capture light and refract it into a rainbow of color.

''The cut makes a stone look as good as it looks,'' said Christopher Jupp, a custom designer at the Dragon's Hoard Jewelers in Champaign. ''If you put a dull stone in a $1,000 setting, you won't have a thing of beauty.''

Jupp said there is a worldwide demand for well-trained diamond cutters and he is glad de Witte decided to take over the school.

Student James Winston of Danville said he got interested in diamond cutting after his brother-in-law brought him rough diamonds from Zaire. Winston, 39, met de Witte and decided to learn the trade, so he would have a new career when he retires from an automotive parts plant.

''Maarten has all the patience in the world and really knows what he is doing,'' he said. ''I can sit there at that wheel for hours and I see my progress and I just don't want to stop.''