Potassium is another element whose name is familiar to most but few have actually seen. When great care is taken to separate a very pure sample of potassium it appears as shiny as the silver backing of a mirror. For that level of conservation you would need to store it under vacuum or an inert gas because otherwise it would very quickly oxidize. Even when stored in inert mineral oil traces of contaminants are enough to spoil the surface finish and turn the metal whitish purple (which is kind of attractive as far as corrosion goes!)
Potassium is also very light and will float on water. Except doing so would be foolish, least of all because it would ruin your precious sample! A metal chunk thrown in water will immediately catch on fire and then explode releasing toxic but admittedly quite colorful byproducts!
Here is a tiny lab-made ampule containing an estimated 100mg of this metal absolutely free of unisghtly blemishes caused by oxidation.