Minerals: Earth’s building materials — Earth science, 11–13 years
Learn how minerals are recognised by properties such as hardness, lustre and crystal shape, rather than by colour alone.
What makes a mineral?
A mineral is a naturally formed solid with a particular chemical recipe and an ordered internal structure. Quartz, salt and diamond look very different, but each has its own repeating arrangement of atoms. Rocks are usually mixtures of several minerals.
Why use properties?
A mineral’s colour can change because of tiny impurities, so it is not a reliable test by itself. Geologists needed ways to identify the same mineral in different places. Hardness, streak, lustre, cleavage and crystal shape give several clues that can be checked together.
Testing an unknown sample
Suppose a clear sample makes a glass scratch, but a steel nail scratches the sample. Glass has hardness about 5.5 and steel about 6.5, so the sample lies between them. Its glassy lustre and six-sided crystals then suggest quartz, whose hardness is 7; the scratch result and the shape agree.
The colour trap
It is reasonable to think that one mineral always has one colour: coloured objects often work that way in everyday life. But quartz can be clear, pink, purple or smoky because of impurities and defects. Use colour as a first clue, then check other properties before naming the mineral.
Minerals in everyday materials
Minerals are useful because their properties make particular jobs possible. Quartz is used in watches and some electronics because it can vibrate regularly, while gypsum is used in plasterboard and halite becomes table salt. Mining and using them also has environmental costs, so materials should not be treated as endless.
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