I was asked this question recently, and my initial answer was “I don’t think so.” To investigate deeper, we need to figure out what a rock is:

• A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral matter. (USGS)

So it looks like we need to figure out what a mineral is next:

• A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. (USGS)

Ice certainly seems to fit these definitions, such that a piece of ice could be considered a rock. To cut to the chase, the International Mineralogical Association does indeed consider ice to be a mineral. And in case you were wondering, rocks can be made of a single mineral (though usually containing impurities).

So the answer to the question is a disappointingly easy “yes,” but I’m more interested in another question: If ice is a rock, why does ice not seem like a rock?

Why ice doesn’t seem like a rock

I think it’s mostly because it melts at room temperature. This is a huge qualitative difference between ice and other rocks. We think of ice as frozen water, and water is not a rock (because it’s liquid and has no stable internal molecular structure).

Ice near its freezing point is also only about as hard as chalk (another rock) and less dense than it, making ice one of the softest and lightest rocks. Consider some of the most abundant rocks on earth: granite, basalt, sandstone, and limestone. The average rock you can pick up off the ground anywhere on the earth’s surface is opaque, grayish or brownish, and very heavy, much unlike ice. As far as I know, humans have always distinguished rocks and ice as different kinds of things.

So, then, what do we do with this information that ice is a rock? I am tempted, as perhaps are you, reader, to file this fact away as a piece of trivia– while, for all practical purposes, maintaining my previous view that rocks and ice are different things. Here’s a different way to approach it.

Science really is telling us something about the world here. What we found is that our definitions of “rock” and “mineral” don’t line up with our intuitions about those concepts. There are a couple ways we could respond. First, we could revise our definitions, for example to say that a rock must be solid at standard temperature and pressure (STP). The problem is that would be ad hoc because the vast majority of the universe (indeed, the vast majority of the earth) is nowhere near STP. Instead, we could simply observe that ice is an extremely unusual rock. Humans are generally uncomfortable with unusual instances of things. However, reality simply does have many unusual things in it. Our thinking can be more in line with reality if we accept the unusual for what it is: a genuine but atypical instance of a concept.

Here’s a fun quiz: What type of rock is ice? Sedimentary, igneous, or metamorphic?