We get this question a lot. Sometimes it becomes apparent by someone’s use of the terms “rock” and “mineral” interchangeably that they aren’t aware of the difference. To know what a rock is, we first have to understand what a mineral is.
A mineral must meet all 5 of the following criteria. Generally speaking, if even one of these criteria is not met, it’s not a mineral. A mineral is:
1) Solid – can’t be a liquid or gas
2) Inorganic - not created by the life processes of organisms (there’s a few exceptions to this; ie mollusk shells are made primarily of aragonite or calcite, chitin, and proteins).
3) Naturally occurring – not human-made
4) Consistent chemically – A given mineral will always have the same chemical formula/makeup
5) Definite in atomic structure – The atoms making up a mineral will always be structured (put together) the same way. In other words, it has a crystalline structure.
Some substances meet all but one of these criteria. Take obsidian, for example. Because it’s a material formed through geologic processes, one might assume obsidian must fall into either the rock or mineral category. We know obsidian forms from quenched, or instantly cooled, silica-rich lava. So wouldn’t it be an extrusive igneous rock or maybe a mineral?
Let’s check obsidian against the mineral criteria – obsidian is solid, inorganic, naturally occurring, and consistent chemically (primarily SiO2). However, being a volcanic glass, obsidian lacks a definite atomic structure. By definition glass is amorphous, lacking a crystalline structure. Hence obsidian doesn’t meet criterion #5, so it’s not a mineral. We would call it a volcanic glass or a mineraloid, the term for a substance found in nature that resembles a mineral.
Now, what’s a rock? A rock is made of two or more minerals and/or mineraloids. That’s a simple definition…but sometimes it’s tricky to determine whether a substance is a rock or mineral by sight. A common misconception is that one can rely on color to determine whether a substance is a rock or a mineral. If we know a rock specimen is made of two or more minerals, it must display two color variations, right? Only a pure mineral sample would be a single color throughout?
Not necessarily…see the photos below. Are you looking at your own rock/mineral collection with some newfound questions? Feel free to reach out to Mama’s Minerals at [email protected] if you feel you need some help identifying your specimens!!
This is Basalt, an often single-color rock composed of at least 2 minerals (plagioclase feldspar & pyroxene).
“Basalt, Lanzarote” by Jamain is licensed under CC BY-SA 4.0
Tourmaline, a mineral which comes in many colors. Two varieties shown, verdelite & rubellite
“Tourmaline-27127” by Robert M. Lavinsky is licensed under CC BY-SA 3.0