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IN THE CELLAR OF Maison Bonnet, on rue des Petits-Champs in Paris, you could almost bottle the concentration. Franck Bonnet, the fourth generation in this family of bespoke eyeglass makers, is slowly turning a pair of buffalo-horn frames, lensless and unpolished, fitting the arms to the sides with rivets. In another corner, a pair of acetate specs is being polished by Leo Fromenteil with paste and a cloth. Boxes, stacked one on top of another, with names like Christian Louboutin and British shoemaker Rupert Sanderson, are waiting to be picked up.

For the past five years, this intimate atelier near the Palais Royal ( ) has been creating Bonnet's bespoke glasses, which start at around €1,000 and go up to the tens of thousands. (The original workshop is still in Sens, where Franck's father, Christian, and brother, Steven, work.)

The sleek upper room displays some of the firm's best-known glasses, including those made for Le Corbusier, Yves St. Laurent and Jackie Kennedy. It's here where clients most often sit down for the initial two-hour appointment to discuss their tastes, habits and professions, and have their facial measurements taken. Next, they choose their color, style and material. Acetate is the most affordable, with frames starting at €950 and coming in almost 30 shades; buffalo horn from Vietnam, India or Madagascar is the middle of the range, with frames available in seven shades starting at €1,400. The most expensive option is tortoiseshell (price upon request), which goes from dark brown to extra blond, the rarest of the lot.

Made up of different layers grafted onto each other, tortoiseshell is so delicate it takes Bonnet apprentices 10 years before they are allowed to handle it. Tortoiseshell was, for the most part, banned by the Washington Convention in 1973. The material the atelier uses was acquired before the ban, and a Master of Art certification from France's Ministry of Culture allows the company to work with the rare material. With such limited resources, stock will, at some point, run out, making these glasses an increasingly rare acquisition.

But obtaining a pair isn't for those in a hurry. Only a thousand frames are made every year, each cut from flat pieces of material before being shaped, filed, fitted together and polished, in either a beautiful shiny smooth finish or a more raw, scratched matte. Glasses made with acetate take two months to finish, while those with tortoiseshell can take up to 4½ months—each layer of the grafted material taking at least one month to dry.

Franck Bonnet scrutinizes the frames he has been working on. Satisfied, he places the glasses in a leather box, ready for the final fitting. Before they are given to the client, each pair is given a Master of Art stamp and is hand-engraved with the owner's name. One customer is so devoted that he has had 30 pairs made over the years. For most people, though, just one of these hand-crafted pairs is enough.