Quick now, can you explain how to tell the difference between a good diamond and a bad one?

Or do you think there's no such thing as a bad diamond?

Or do you stay away from diamonds altogether because their prices these days resemble what you once paid for a house?

If shopping for diamonds makes you nervous and vulnerable, you're not alone.

Even people whose jobs depend on knowing the diamond market find spiraling diamond prices bewildering.

"Anybody who tells you," says a diamond wholesaler, "they know what a stone is worth now is lying." The price can fluctuate as much as 10 percent in a month.)

Not only have diamond values in general flashed upward in recent years, but prices for diamonds of the same size are wider than ever. A quarter-carat diamond can cost from $600 to $2,000; a half-carat from $1,300 to $7,100; and a full-carat, $2,000 to $78,000.

Despite the confusion, however, area jewelers say diamonds are still the favored purchase for engagement rings, "status" gifts and "investment jewelry." Knowing what can account for the difference in prices -- along with some comparative shopping -- can make the diamond hunt consideraby less mystifying.

Armed with my knowledge of the "FourC's" (see box) I visited Melart Jewelers at Tysons Corner to do some diamond "shopping."

Salesman Jim Davis spread three diamonds, all in the half-craft range, on the counter. One was clearly the most brilliant, even to my naked untrained eye. Davis said it was flawless. Price? $7,100.

Using the flawless diamond as a huide, I was able to identify which of the remaining two was more valuable. One had a distinctly yellowish (almost dull by comparison) tinge to it and turned out to be JK grade stone, retailing at $1,300.

The mid-priced stone ($2,700) was noticeably less brilliant than the flawless one. But I must admit, without the flawless diamond there for comparison. I would have had difficulty making a distinction between the midpriced and the lowest-priced stone.

Area jewelers say rising diamond values have made customers more cautious about purchases, but sales are still good.

"The economy has had some effect, even in Washington," says E. I. Kaufmann, owner of the Maryland Diamond and Jewelery Exchange at Lohmann's Plaza, Rockville. "In the past, a person might have $5,000 to spend on a diamond; today it might be a little less."

But not for everyone.

For the "very finest one-carats available -- flawless, colorless and cut perfectly," Kaufmann says the price is $78,000 each.

Orders are filed individually, and there's a waiting list.