ON A BUSTLING STREET in Paris’s Marais neighborhood, the gay enclave that feels symbolic of the city’s free-spiritedness, there’s a sober pair of 17th-century wooden doors. Once, they opened onto a space that belonged to a group of nuns. Now, though, they allow entry into the sumptuous and hushed sanctuary of Betony Vernon.

A designer of erotic jewelry, a self-described “sexual anthropologist” and the author of the candid yet refined 2013 sex guide, “The Boudoir Bible,” the 49-year-old Vernon is a jeweler turned sex educator. She holds sessions in her private home studio in which she helps clients find erotic awareness through a combination of talk therapy, movement and occasional treatments in the nine-foot bronze, steel and leather swinglike contraption that hangs from the ceiling of her basement. A client climbs in and is suspended horizontally four feet above the ground for 25 minutes — Vernon, a certified medical hypnotist, uses this version of her “theta rig,” which she designed, to ease the person into a meditative “theta state,” the brain-wave frequency reached during yoga and orgasm.

Vernon, who moved to Europe from Virginia in 1990, believes that Western culture no longer has many taboos around sex, but “rather a taboo against pleasure.” Her three-floor apartment, a sensual mélange of velvet and leather, is a rejoinder of sorts: a design manifesto for aesthetic pleasure, both visual and tactile. Entering it from the row of beige-stone facades on the street is like cracking a plain gray rock and finding a sparkling geode inside; you can’t quite believe this treasure is nestled in the workaday heart of a city.

“It’s my little Eden in the center of Paris,” Vernon says, referring to the central garden onto which the apartment looks. Eden is also what she calls the first-level salon, where she receives clients for bespoke jewelry fittings. With its black carpet strewn with pale-green faux leaves and forest-green velvet curtains that shroud the windows completely, the room feels like the plush interior of a jewelry box. In a sense, it is. Here, pieces of Vernon’s “sado-chic” erotic jewels, as she calls them, referring to both their ornamental and amorous functions, are displayed in backlit vitrines like works of art: a sterling-silver cuff with a detachable feather tickler, an equestrian-inspired necklace that can be worn as part of a harness.

Of the black-and-green scheme she chose for the room, she says: “I wanted to do a color that was unisex, that made everyone feel comfortable, and I thought green was good, because it’s the color of trees.” The natural world and its palliative force is further evoked by a lime-green velvet Gio Ponti chair that vaguely resembles a pea pod, and a black velvet Mark Brazier-Jones chaise longue with silver steel satyr feet. Two Fornasetti pieces — an ocher-hued commode painted with a prowling leopard, and a screen depicting a verdant forest in a dark night — complete a visitor’s sense of being deep in a lush jungle.

But beneath Vernon’s specific fantasy is another increasingly common one: the desire to leave behind the quotidian world and its pedestrian concerns, at least for a moment. There are no cellphones allowed in Eden (and no Wi-Fi in the basement level she calls Heaven, where she holds her sexual well-being workshops), and the functional aspects of life, like computers and printers, are hidden from view. In the corridor leading to her studio, where she prototypes her designs (which are produced by artisans in Tuscany) she has hung a black leather curtain that allows her assistants to stash their clutter at day’s end. “I don’t like to look at work things,” she says. “Offices are never attractive.”

VERNON ADHERES TO this cloistered philosophy in her living quarters as well. In the retro-pink kitchenette — a pale, matte, ’50s-era shade she chose because she wanted the room to feel like a “boudoir kitchen” — all of the appliances slide behind wooden doors. Her inspiration here (and elsewhere in the 1,200-square-foot space) was the snugly efficient design of yachts: “A lot of the furniture is built-in and customized to get the most out of every centimeter I had at my disposal.”

When she purchased the townhouse in 2007, it was actually far smaller. Only the basement and the room where Eden now is were hers. She was living in Milan, where she’d obtained her master’s in industrial design and founded an atelier, commuting back and forth to Paris. But in 2011, when the adjacent apartment became available, she bought it, renovated the kitchen and hired local craftsmen to restore the original parquet floors and carved wood spiral staircase.

On the top floor, Vernon created a cocoon-like suite for herself in rich, ultra-feminine pastels. There’s the sitting-room library, with its book-lined lilac walls and plum-colored Nigel Coates divan. There’s the bedroom, padded entirely in leather of the palest ballet-slipper pink and lit only by crystal-and-metal Mark Brazier-Jones fixtures — a kind of sealed-off sensory-deprivation chamber. “I padded everything, including my blinds on the windows,” Vernon says, “so when it’s closed up at night, it’s very quiet and safe-feeling.” And finally there’s the “wet room,” lined in dusty-rose and purple Moroccan cement tile, where Vernon installed the most capacious two-person tub she could find.

The solitude and restoration these rooms offer are vital for her, because back downstairs in Heaven, the fourth-century vault that is the oldest part of the building, the work Vernon does with clients is intimate, delicate and intense. This room is dominated by her “boudoir box,” a two-foot-tall black leather case, where 21 of her “jewel tools” and up to 80 pieces of her Paradise Found fine-jewelry collection are kept under lock and key. Vernon created her own series of implements — collars, massage rings, tassels, whips — after years of frustration with sex toys that, as she puts it, “satisfied neither my lust for quality materials nor my sense of aesthetics.” The box is one in a limited edition of 10 that she designed 18 years ago (when major luxury retailers were still reluctant to carry her work), so that she could travel with her tools to clients. Since May, an entire set has been on display at the Paris Museum of Modern Art.

“The box gets opened, and it transforms the room,” Vernon says. It is the showpiece of the small cavelike space, surrounded by a bio-ethanol fireplace and leather seating that can be separated into various components to accommodate up to 15 people. “I do my interiors like I do my jewelry,” Vernon says. “A piece cannot just be beautiful. It must also function.”

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Photography by Thibault Montamat