A large ornamental piece of jewelry, triangular in shape, worn by a woman on a bodice and extending from the décolletage or below to the waistline. Such pieces were usually of floral or ribbon design,set with many small gemstones and sometimes with several pendants. Some were made sectionally so that they could be separated into two or three parts, to be worn individually. They were used in the18th century and later, especially during the Edwardian period. Sometimes called a devant le corsage or a 'corsage brooch'.
Stomacher

Stomacher
A large ornamental piece of jewelry, triangular in shape, worn by a woman on a bodice and extending from the décolletage or below to the waistline. Such pieces were usually of floral or ribbon design,set with many small gemstones and sometimes with several pendants. Some were made sectionally so that they could be separated into two or three parts, to be worn individually. They were used in the18th century and later, especially during the Edwardian period. Sometimes called a devant le corsage or a 'corsage brooch'.
References
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson



