I found this story in Nancy Armstrong’s Victorian Jewelry (1976).
In the 1850s and 1860s, as skirts widened, they had to be balanced by the lowness of the neckline on evening dresses. So evening gowns went from this in the 1840s:
where the neckline stopped not far under the clavicle, to this:
in the 1850s and 1860s, where you got more than a hint of cleavage. As such, jewelry and other fashions had to change to match these new bosom-baring styles.
In particular was something called a “bertha”, or “bertha collar”, which was a border around the low collar of a dress (it could either be fixed to the dress, or removable). It was “composed of ribbons, ruches, laces or embroideries and trimmed with flowers and feathers, giving play to any amount of flights of fancy” (60). Berthas had been around for a while, but they became more integral to formal wear for women at this time.
Some examples of berthas include:
“Quite the most unique was a bertha made for the Empress Eugénie of France, almost entirely from rubies, sapphires, emeralds, turquoises, amethysts, jacinths, topazes and garnets (the favourite nineteenth-century gemstones) linked together with the Crown Jewels.
“Certainly the general exposure of the neck and shoulders might have been startling to a newcomer and it is no surprise that a provincial, who was invited to a ball given in the Tuileries in 1855, said in disgust that ‘he had never seen such a sight since he was weaned’” (60).