Re: HOW DO YOU FIND THE HARDNESS OF ROCKS

Dear Melesio,

First of all, we do not measure the hardness of rocks.  Instead we measure 
the hardness of minerals.  Rocks are made of minerals, and a rock usually 
contains more than one mineral.  Because minerals differ in hardness from 
one to another, it does not make much sense to measure the hardness of a 
rock that contains several minerals.

The hardness of minerals is measured using a scale devised by a 19th 
century German mineralogist named Friedrich Mohs.  Mohs used a scale of 
hardness from 1 (the softest) to 10 (the hardest), and the scale assigns a 
mineral or other material to represent each hardness.  So if you want to 
find the hardness of any mineral, you compare it to the minerals on Mohs’ 
scale.

Mohs’ scale is as follows:

1 –  talc
2 –  gypsum
2.5 –  a fingernail
3 – calcite
3.5 – a copper coin
4 – fluorite
5 – apatite
5.5 – window glass
6 – feldspar
6.5 – hardened steel (as a knife blade)
7 – quartz
8 – topaz
9 – corundum (for example, sapphires and rubies)
10 – diamond

The way to use Mohs’ scale to determine the hardness of a mineral is to 
see which minerals on the scale will scratch it, and which ones it will 
scratch.  For example, if a mineral will not scratch quartz then it must 
be softer than hardness 7, and if it will scratch calcite then it must be 
harder than hardness 3.

Be careful when you use the scratch test to make sure you know which 
mineral has really been scratched.  If you scratch a softer mineral 
against a harder one, the softer one will usually leave a powdery streak 
on the harder one.  For example, talc would leave a streak on a copper 
coin, but if you brush away the powder and examine the coin closely you 
will see that the coin has not been scratched.  In this example, 
therefore, talc is shown to be softer than copper.  It helps if you have a 
magnifying glass so that you can examine minerals closely after using the 
scratch test.

Even if you can’t get samples of all the materials on the scale to use for 
testing, you should be able to get several of them easily – a copper coin, 
window glass, a steel knife and your fingernails are easy to find.  Quartz 
and calcite are very common minerals, and quartz is the hardest mineral 
you are likely to find in abundance.

Diamond is the hardest natural mineral, although materials harder than 
diamond have been manufactured.

I hope this answers your question.

Best wishes,

David Scarboro