The suspension loop suggests that the brooch was originally a detachable pendant to a sautoir, for which this is a typical design.
Aquamarines play one of the most important roles in the designs of Cartier during the 1930s. Of the twenty-seven tiaras made by Cartier London in 1937 with the coronation of George VI in mind, most were set with aquamarines and diamonds or topaz and diamonds.
Aquamarines (as well as topazes) were considerably more economical than diamonds and gave Cartier the opportunity to experiment with the use of large stones and create bold designs. At their best, aquamarines display a fresh light blue untainted by a greenish tinge. The problem of obtaining stones of good colour becomes apparent in a letter from Cartier London to Cartier New York in December 1936: 'We would mention that owing to the difficulty of obtaining quickly a supply of good colour aquamarines, we are unable to fix a definite date for completion of the order, but we do hope to deliver in America in 2 months from receipt of order. As a matter of fact, we have for our own stock a number of partly completed necklaces, which we are unable to finish owing to lack of the necessary aquamarines.'
According to Judy Rudoe in Cartier: 1900-1939 (London 1997), p. 263,
'Much of Cartier's aquamarine jewellery seems to have been made by the London branch, where it appears in the records from 1932. Aquamarines were popular not only with the London clientele but also with the American clients of both the London and Paris branches. ... Another American client, Elsie de Wolfe (then aged 70), commissioned an aquamarine tiara from Cartier Paris and, true to form, had her hair tinted to match it.'