Based on Fiona and Cake
Crystal weapons turn into crystals when not used. Crystal swords and other one-handed weapons turn into about fist-sized crystals and can be stored as jewellery and concealed. Crystal daggers turn into even smaller crystals that can be worn as part of a necklace or bracelet. Making them in different colours is trivial, thus all shades are available. They usually activate by holding them in a specific way, but sometimes they are rigged to react to emotions like rage or fear. They tend to be as powerful as their mundane counterpart, but cost at least as much as the jewellery they pose as, so they are mostly used by rich ladies as a safety measure.
Disclaimer: the “items of” series is in no way affiliated to it’s source media. The descriptions are entirely unofficial and sometimes contrary to the original intention on purpose. No claim of ownership intended.