Stomachers are the front triangular panels in the bodices of women's gowns. They were popular in the 15th -18th centuries. They were often heavily embroidered, ribboned or bejewelled.

One famous fan of stomachers was Queen Mary (1867-1953), Consort to King George V, and grandmother to the Queen Elizabeth II. A regally poised woman, she had her own sense of style and loved jewelry. She wore lots and yet got away with it much to the admiration of her contemporaries. She always dressed up for dinner every evening, complete with tiara and carried jewelled fans. Her famous all diamond stomacher shown in the picture was an elaborate three tiered affair so her evening gowns had to be reinforced with buckram in the front to bear its weight when it was pinned on. Queen Mary's interest in jewelry and involved planning played a large role in today's Royal Collection.
The photograph was taken in Ottawa in 1901when she was still Princess Victoria Mary, Duchess of Cornwall and York on a Royal Tour of Canada with her husband. Notice her tiny waist - corseted 18-inch waistlines must have been so uncomfortable!

Leslie Field (1987). The Queen's Jewels : The Personal Collection of Elizabeth II. Harry N Abrams Inc.