What is the significance of Monsieur Regnault’s diamond pin in La Grade Bretche?Why does he mention it to Dr. Bianchon?
One could argue that the significance of Regnault's diamond pin in "La Grande Bretech" is loyalty. Regnault states to Bianchon that he was given the diamond by the Countess on her death bed.
The diamond is first introduced in the description of Regnault when Bianchon first meets him. Regnault is described in the following way:
The stranger wore an old coat, much worn at the seams; but he had a diamond in his shirt frill.
Later, the story about how Regnault came to posses the diamond is the story of the tragic Countess de Merret. Rumors had lead Regnault to believe the Countess to be "a coquette." Only after meeting her did Regnault learn the truth about her.
What Regnault sees in the bedroom of the Countess forces all negative thoughts from his head. Regnault begins to feel great emotion and sorrow for the woman.
The diamond was a gift from the Countess that reminds him of his oath to her as her attorney and executor. The most telling quote in regard to the diamond's meaning is when Regnalut rejects Bianchon questions about the Countess' will:
I never allow myself to criticize the conduct of a person who honours me with the gift of a diamond.
What this shows is that Regnault is devoted and loyal to the Countess; she can trust him even after death and he will not question she who has honored him in such an extraordinary way. The diamond represents his loyalty and devotion.