Jean-Baptiste Fossin was born in Paris on 28 June 1786. He was foreman of the workshop of Marie-Étienne and François-Regnault Nitot, jewellers to Napoleon, and when François-Regnault withdrew after the emperor's fall in 1815, Fossin bought the business. His son Jean-François, known as Jules (1808–1869), joined him, according to French Wikipedia, in 1830.
Father and son became known for romantic jewellery inspired by the Italian Renaissance and the French eighteenth century, and for naturalistic pieces: vine leaves, fruit and hawthorn, ivy, olive and chestnut leaves, eglantine, jasmine and geranium, often with green enamel leaves for realism. From a shop on the rue de Richelieu they served the nobility of the Faubourg Saint-Germain and the bankers of the Chaussée d'Antin, the family of King Louis-Philippe, the Duchesse de Berry and Prince Anatole Demidoff, as well as foreign residents such as Princess Bagration and the Duchess of Galliera. In 1837 they made pieces for the wedding trousseau of the Duke of Orléans. Fossin is credited with the emerald and diamond Leuchtenberg tiara, which can be taken apart into a brooch and six hair ornaments; Wikipedia links it to Queen Hortense. From 1834 his workshop was run by the goldsmith Jean-Valentin Morel.
Fossin was also a painter and sculptor: he showed at the Paris Salon in 1846 and 1847, with works including Le Triomphe du Christ and a marble La Prière, and won a bronze medal in 1847. He died at Vaudoyen on 4 October 1848 and is buried in the Montmartre cemetery. After his death Jules went into partnership with Morel in London; Morel's granddaughter later married Joseph Chaumet, who gave the house its present name.
The attribution of the Leuchtenberg tiara to Queen Hortense comes from English Wikipedia and has not been checked against a museum record.