"It's a horrible problem," said Edward Munvies of James Robinson, New York, noting that sometimes a reproduction is evident to the eye by the rough piercing of the setting.

Joyce Jonas, an appraiser who teaches a course in estate jewelry at New York University, advised prospective buyers:

"Examine the setting. A dealer may not be aware of a stone replacement, and you may be able to feel a roughness around the collect," the collar that holds the stone.

Pieces with smaller stones in pave settings are most likely not to have been tampered with and are your best bet if you want things in absolutely original condition.

The savvy amateur buyer should seek expert advice from an independent appraiser, for example, who would examine a piece on-site - in a dealer's shop or at an auction preview - to determine that at least the stones are in fact diamonds.

Judging purity of design requires another kind of connoisseurship.

The carat weight of individual stones cannot be precisely determined without their removal from the setting, which is certainly not recommended with old pieces. Irreparable damage might be done.

The variety of antique diamond jewelry is wonderful and expensive. At one salesroom a minuscule diamond roadster, circa 1900, with wheels that turn, was recently priced at $8,500. In the same display case, a sparkling flower basket the size of a quarter could be had for $7,500.

At the Ralph Lauren mansion, an exquisite oval brooch (circa 1919), glittering with pave-set diamonds and sapphires, was all set to dress a neo-Edwardian lapel for $9,300.

In the diamond market, stones are assessed on the basis of inherent perfection. Antique diamond jewelry is appraised for style, quality of workmanship and condition more than for the overall quality of the stones.

"Sometimes humble materials are worth more than precious, primarily because of the way they've been worked," said Peter Schaffer of A La Vieille Russie, which sells Russian antiques and Faberge bibelots. A diamond mouse with garnet eyes, circa 1800, will set you back $8,500 there.

Many conventional forms, flower sprays, stars and bows date back to the 18th century, when diamond cutting and setting preoccupied the European jewelry industry. The natural sciences encouraged designs of bees, butterflies and flowers.

By the middle of the 19th century, diamond jewels like corsage ornaments and tiaras reached spectacular proportions.