When a koyukkuri finds an object she favors, sometimes she would declare it her "treasure" and take it home. The treasure can be anything but it's usually a pretty rock or a shiny object such as a marble or a coin. A koyukkuri's treasure will make her feel easy just by touching or rubbing it, but they often have no actual value.
Despite most koyukkuri treasures having objectively no value, koyukkuris value a treasure so much that it can cause koyukkuris to get jealous over another's treasure, make them bicker and fight over the ownership of a treasure, and if they're a shithead they might even kill eachother just to steal a treasure. If the treasure happen to originally belong to a human (such as a dropped coin) it can also make the koyukkuri try to fight the human to defend their treasure (instead of just running away) when the human tries to take it back, which usually leads them to their death.