A Kirara Shell (キララ貝, Kirara Gai) is a gigantic shell that is able to produce shining and sparkling light from its lower shell by absorbing sunlight and moonlight. It is located in the Coral Forest.
Sitting in the Kirara Shell's shining and sparkling light will make you feel cozy, warm, and sleepy.
The shell closes once every 3000 years, and during the time that its closed, the Kirara Shell's light will break down and turn into a bunch of shining pearls that behave like bubbles, and then the Kirara Shell will spit them out, once it opens back up.
Atagoal Monogatari Chapter 11 - Taking a Nap in a Kirara Shell (キララ貝でひとねむり)