Ice is a perfectly respectable mineral, albeit one with an unusually low melting temperature of about 32 degrees Fahrenheit or zero degrees Celsius. Because ice is a mineral, an aggregate of ice crystals is a genuine rock capable of melting, as with liquid mercury, at room temperature. When the liquid water of a river freezes, it becomes a transient igneous rock that is destined to be broken up and rafted downriver like miniature tectonic plates floating on a denser fluid below.