JAR emerald brooch. 

The form, scale and narrative of this imposing brooch and the ingenuity of its craftsmanship show the brilliance of Joel Arthur Rosenthal of JAR, the most influential jeweller of modern times. The evocative architectural silhouette contrasts with the crisp form of the rare antique emerald. Amazingly, the emerald is held in place between two slices of rock crystal over carved white agate. Intricate soft-gold openwork on the back of the brooch replicates an Indian jali, the type of screen through which at one time the women of the Indian court would watch ceremonies or comings and goings in public spaces.

(source: Beyond Extravagance: Gems and Jewels of Royal India (Assouline)