Mr. Brown's Science Labs

How Geologists Identify Unknown Minerals

A mineral is a naturally occurring solid with a specific chemical composition and a regular crystal structure. Earth has over 5,000 known minerals, and they all look different. So how do scientists tell them apart?

Geologists run a series of physical tests on a mystery rock. The most important test is hardness, measured on the Mohs scale, which ranks minerals from 1 (softest, like talc) to 10 (hardest, like diamond). A harder mineral will scratch a softer one, but never the other way around. Geologists test hardness with everyday objects: a fingernail (about 2.5), a copper penny (about 3), a steel nail (about 4.5), a glass plate (about 5.5), and a steel file (about 6.5).

The streak test is another favorite. When a mineral is rubbed across a rough white tile called a streak plate, it leaves behind a powdery line. The color of that powder is the mineral's streak — and it can be very different from the surface color of the rock. For example, pyrite looks gold but leaves a greenish-black streak, which is why it cannot fool a geologist!

Some minerals have unique properties that make them easy to spot. Magnetite is the only common mineral that strongly attracts a magnet. Fluorite glows bright purple, blue, or green under ultraviolet (UV) black light — a property called fluorescence. Calcite bubbles and fizzes when a drop of dilute acid is placed on its surface. And halite, which is the same mineral as table salt, dissolves completely in water and tastes salty (do not actually taste mystery rocks!).

By combining several tests, geologists can identify almost any mineral — and so can you. In this lab, six mineral suspects are waiting. One of them committed a (very minor) crime in Mr. Brown's classroom. Time to investigate!