NEW YORK--It's called the Rattle Ring, although it's obviously not for babies unless Baby happens to be chic, stylish, au courant and more concerned with diamonds than diapers. The diamonds, you see (or, more accurately, don't see), are hidden inside this ring. Talk about surprises!

The brainchild of John Reinhold, a New York diamond dealer, and Marc Jacobs, a fashion designer, it is certainly different from other rings around town. On the exterior, this one is a simple-looking two-piece tubular band of 18-karat yellow or white gold.

But wait! Rotate the inner part of the closed ring against the outer part and voila! a small square portal slides open, revealing eight tiny compartments, each perfectly sized to hold one loose quarter-carat diamond. Even those secret decoder rings in old-time comic books never offered quite such a surprise.

The name Rattle Ring, Reinhold explained, comes from the "music made by the diamonds dancing within their little houses as the ring is moved." The more diamonds within, the more apparent the music. Most people, he said, start out with four jewels, adding others as occasions demands sort of a charm bracelet or add-a-pearl of the '90s.

Reinhold, in fact, thinks of the ring as being "very 90s, discreet rather than ostentatious," like wearing a raincoat lined with fur or having a monogram hidden on a sleeve rather than displayed on a cuff.

They can, of course, be costly, too. A Rattle Ring with two diamonds (the minimum) is $3,250; 4 diamonds, $5,250, six diamonds, $7,250, and eight diamonds, $9,250.

The hand-made rings may be purchased by appointment from Reinhold in his showroom at 555 Fifth Avenue (212) 867-8315.