Whoever you are out there, John Gurney wants you to know that the diamonds you stole are worthless.

They look like cinders, not diamonds, don't they? There even is a $200 reward if you'll just give them back, please.

Gurney, a professor of geochemistry from South Africa who is visiting here, spent three years collecting his small pouch of monetarily worthless, throwaway diamonds, a type usually destroyed in the diamond-mining process. It is the impurity of the crystals that fascinates Gurney.

On Thursday, at Dulles airport, he picked up an eight-by-eight-inch box, wrapped in linen and marked with wax seals saying Diamond Trading Corporation. The little pouch contained the only good samples in the world of the impure diamonds, those with minute fusions with other crystals.

But over the weekend they were stolen from Gurney's temporary home in Silver Spring.

"I don't care how I get them back," said Gurney. "I'm not interested in who took them. And whoever returns them, I'm not interested in how the person explains how he got them. He can give them to me or to The Washington Post, or whatever. I just want to get them back."

They are a scientific oddity because diamonds are probably the most naturally pure substance on earth, pure carbon crystallized at huge pressures and high temperatures about 100 miles below the surface of the earth.

Among all the crystals, diamonds alone very rarely fuse with crystals of other minerals when they develop.

Gurney has -- or perhaps had, if he can't get his diamonds back -- a four-month research grant at the Carnegie Institution in Washington to study the 250 cinders. It is not known at what precise temperature and depth diamonds are formed, and why they are so pure. But other, softer crystals, such as Gurney's, can be tested to answer those questions.

So after three years of collecting, John Gurney came to Carnegie to study them. He worked Friday and Saturday. Since he was alone in the Carnegie laboratory on Saturday and didn't know where he might store them, he brought them to his temporary residence in Silver Spring.

When he left for the weekend, he put the little pouch under the base of a pedestal. On top of it were several drawers and a heavy clock -- a couple of hundred pounds of deterrence.

But someone seemed to want only those diamonds. More than $1,000 worth of camera equipment was untouched, and the burglar apparently searched only in places likely to hold a small pouch. The burglar dumped out a box of raisin bran, for example, and opened all the drawers in the place, before leaving with the bag of rare cinders.

Gurney speculates that someone overheard a conversation about the diamonds being studied, and thought that they were valuable.

"I haven't got much money," Gurney said, "but I can pay $200 to get them back."