Magatama Idol Kaguya explains:Edit
"Unlike the stone known as Pumice which floats on water, this stone floats on its own! This is the stuff that keeps Kyoto afloat... It's so soft and fluffy! If you're going to put it into your bag, you better make sure to properly attach a string otherwise it'll float off! Here, look how I tied mine... Oh... It's gone. "