from the little we can see of this Brooch/locket, it certainly does look beautiful indeed and of French origin. Brooch lockets were popular during the early to mid 1800s. Early lockets were used as keepsakes for a lock of hair, later in the century, with the invention of photography - a photo. The clasp is a "C clasp" and is the earliest type of catch.
BREVET S.G.D.G. is what's known as the sponsor mark or responsibility mark, and it's the initials of the firm or indiviual who submitted the piece for marking. Brevet S.G.D.G. is the French patent mark from 1844.
I noticed you are from Holland Marga, and the Dutch, who are members of the International hallmarking Convention, have been striking hallmarks since at least 1814.
The Dutch have publish a book entitled "Netherlands' Responsibility Marks since 1797" (in three volumes and in the English language) illustrating all the responsibility marks registered there since that time.