It's clearly this same brooch worn by Queen Elisabeth
Christophe Vachaudez writes in his book "Bijoux des reines & princesses de Belgique" on the subject of wedding gifts: " ... and princess Marie Gabrielle of Bavaria gives her a clover leaf set with three cabaochon cut emeralds" (p. 94, my own translation)
Marie Gabrielle of Bavaria was Elisabeth's sister.
So this brooch dates from 1900, or at least that's when it was given to Elisabeth.
Does this mean this is the piece of jewellery that's been in the Belgian RF the longest? Other pieces may be older, but have been acquired at a later date than 1900.