BOOK REVIEW : Deft Tour of Exotic, Unexplored World : DESIRE <i> by Amy Wallace</i> ; Houghton Mifflin $19.95, 223 pages

Frail, neurasthenic Lily Van Velsen is a gemologist who specializes in the cutting of pearls, an exceedingly delicate art at which her great-uncle Mordecai was the grand master. Now that even the most costly pearls are cultured by introducing a bead into oysters’ shells, pearl cutters are virtually extinct; their services required only when a rare antique gem must be matched.

By peeling away the multiple skins of a natural pearl layer by layer, these highly trained experts can uncover a series of tints--rose, green, blue, cream--each exquisite, but only a phase in the revelation of the hidden pure-white globe.

One slip of the miniature tools, one minuscule step too far in the search for perfection, and a magnificent pearl could dissolve into gray dust.

Pearl cutters learn to contemplate before they cut, visualizing what lies beneath each successive color. Such recondite and fascinating lore has clearly captured the imagination of author Amy Wallace, who uses it as the matrix of “Desire,” a novel as multilayered and mystical as the process itself.

Mordecai Van Velsen, the aristocratic semi-invalid old man who lives with his equally ancient manservant, Chen Li, in a musty mansion in San Francisco’s Pacific Heights has retired from Van Velsen Jewelers. Now he devotes himself to literature and philosophy, leaving the still-flourishing concern in the capable hands of Alan Purdue, Lily Van Velsen’s devoted but stolid fiance.

Orphaned in early childhood, Lily was brought up under the attentive supervision of her great-uncle, growing even closer to him as she matured; becoming his spiritual as well as his professional heir. She visits often, never leaving without an armful of books selected--actually assigned--by her great-uncle.

When the novel opens, her reading material includes transcripts of letters written in 16th-Century France; fragments of Van Velsen family history. At that time and in that place, a young woman who became pregnant by an unsuitable young man was dispatched to a convent where she eventually went mad and died of thwarted love and grief.

Thoroughly modern Lily is overcome by a psychic affinity for this tragic creature, and on an otherwise routine errand for Van Velsen Jewelers, she herself falls victim to a sexual obsession.

The object of her passion is an arrogant sculptor with the power to awaken erotic desires never aroused by her gentle and affectionate fiance. Completely unstrung by her enslavement to the sculptor, Lily is afflicted by symptoms eerily similar to those of the 16th-Century French girl. In the throes of her anguish, unable to eat, sleep or work, Lily trails her great-uncle’s manservant on his mysterious rounds of the city, finally to an opium den.

This discovery leads her to a bout of opium addiction, and eventually, by an extremely circuitous route, to aspects of her great-uncle’s history never suspected. Along the way, we’re regaled with Mordecai Van Velsen’s adventures as a young man, compelling in themselves and enlarging greatly our already expanded knowledge of pearls, but doing little to give this ambitious novel the cohesion it desperately needs.

Just at the point when the entire invention might shatter like an overly worked pearl, a fragile connection is established.

Though the realistic scenes in contemporary San Francisco are only functional, Amy Wallace is wonderfully adept at the far more demanding job of evoking the atmosphere of medieval France, re-creating the remote world of pearl divers in Ceylon, and imagining the atelier of the pearl maestro where Van Velsen learned his skills.

There, in exotic and unexplored territory hundreds of years and thousands of miles from Van Velsen’s shop at Maiden Lane and Grant Avenue, the author is truly at home.

Next: Carolyn See reviews “Splinters” by Erica Heller (William Morrow).