First off, the treasure in question. Louis XIV of France was married to his cousin, Maria Theresa of Spain. They gave their firstborn son the imaginative name of Louis. This son Louis was known as the Dauphin of France, i.e. the heir apparent. (I have no idea why "dauphin," the French world for "dolphin," means "the King's oldest son," but there is much I don't understand about French.)
Anyways, the Dauphin never made it to King--his dad outlived him--but while he was waiting, he managed to amass a wonderful collection of "sumptuary items" that collectively became known as "The Treasure of the Dolphin."
Many of the surviving items of this treasure are in the possession of the Museo Del Prado. So, unusually, are the cases designed to hold the individual pieces, from the 1500s and 1600s. And they've gathered many of them alongside their contents for this exhibition.
The vast majority of the cases that protected the works of the Treasury have been preserved, some very luxurious, and all unique specimens, designed specifically and individually, so they may well be called the other Treasure, given their artistic historical value and uniqueness.
The cases not only preserved the works, but also allowed them to be moved safely and facilitated their counting in the palaces' jewelery boxes, as they were easily recognizable because they were adapted to their shapes and design.
While visiting hours are still affected by COVID, Madrid's Museo Nacional del Prado has thoughtfully made the exhibition available online. You can check out high-res images of these unusual pieces here.