Lasque diamonds are flat diamonds with irregular outlines, a cut that originated in India centuries before. Also known as portrait diamonds because they were used as covers for miniature portraits, lasque diamonds are a niche strand of diamond collecting.

The flat shape of a lasque diamond makes it suitable for a watch crystal, which is what Patek Philippe did with a 13.43 carat lasque diamond in the early 1990s. As the story goes, the flat diamond was handed over to the Geneva watchmaker in 1990 by an anonymous client (in fact, industry lore has it there was more than one such diamond).

Over the course of the next year, Patek Philippe constructed a wristwatch around the triangular stone, creating an asymmetric watch case in white gold to fit the diamond. The watch was completed in 1991, and delivered to the client in January 1994 according to the archive extract that accompanies this watch.

Fitted to a fixed, integral bracelet in white gold, the case is a one of a kind creation, a true bespoke watch, created from the ground up for the client. Correspondingly the watch has a one-off model reference: ref. 3843/1.

Patek Philippe infrequently creates unique references, making this a notably rare watch. Intriguingly, a one-off titanium Calatrava with a 9.44 carat diamond set into the back, doubling as a display back of the watch, was sold at Sotheby’s two years ago for US$737,000.

Measuring about 32mm by 26mm at its extremities, the lasque diamond is not immediately recognisable as a diamond. Only the facets on the edges and the inclusions visible at certain angles give it away.

Its weight of 13.43 carats ranks the stone amongst the largest lasque diamonds know, most of which are portraits diamonds in European museums. The largest is the 25 carat stone with a portrait of Emperor Alexander I of Russia on display at the Hermitage in St Petersburg.

The size of the diamond means the watch is larger than it appears in pictures, being some 31mm in diameter. The styling is slightly reminiscent of 1970s watches; the sleekly asymmetric watches designed by Geneva jeweller Gilbert Albert come to mind though the 3843 is significantly more straight-edged.

The ref. 3843 beside a 37mm Langematik Anniversary

The dial is a restrained blue with a metallic finish, while the movement inside is the calibre 177, a slim hand-wound movement. More unusual is the inside of the case back, which is marked not only with the reference and serial number, but also the weight of the lasque diamond that doubles as the crystal, as well as the year of production.

This is not the first time the ref. 3843 has come up for sale. In fact, the same watch sold in early 2015 at a Japanese auction house’s sale in Hong Kong for HK$767,000, or about US$98,900. The jump in value illustrates the value of selling at an international auction house armed with strong marketing.

The Patek Philippe ref. 3843 is lot 2922 at Christie’s Hong Kong sale that takes place on November 28, 2016. It carries an estimate of HK$2.8m to HK$4.8m, equivalent to US$363,000 to US$622,000.