In geology hardness has a special meaning.
It is the amount of force required to scratch a
mineral. But most people who aren’t geologist
don’t think of hardness this same way, so I will
give you two answers. The first is the geology
definition of hardness. The second is why some
rocks are more solid than others.
Mineral hardness (the geology
Because it is really hard (and impractical) to
measure force like this, we typically use a
relative hardness scale: “This scratches quartz
but not diamond. So it is harder than quartz, but
softer than diamond.” We call this relative
hardness scale the “Mohs Hardness Scale”
and it is just a list of ten minerals in order of
how difficult they are to scratch:
1) talc (baby powder)
2) gypsum (a kind of salt)
3) calcite (the same stuff some snails and
clams make shells out of)
4) fluorite (another kind of salt)
5) apatite (your tooth enamel is made of a
variety of apatite)
9) corundum (rubies and sapphires are what we
call gem-quality corundum)
Most people don’t carry all these around with
them, so a quick way to test hardness is this:
your fingernail is 2-3. Steel (like a pocket
knife or nail) is usually 5-6, glass is about 7.
The reason minerals have different hardness is
because the forces that hold their atoms
together are different. In some minerals (like
diamond) all of the atoms are held together so
strongly, that it takes a lot of force to
break them apart. In other minerals, some of
the atoms are only weakly connected so it is very
easy to break them apart; the graphite in your
pencil is a good example. It breaks apart when you
rub it against almost anything, which is why it is
good for pencils!
Why some rocks are harder or more solid than
In general, how strong a rock is depends on
how much empty space there is in the rock and how
strongly all the pieces of the rock are held
Some rocks are very hard to break, because they
are made of crystals that have all grown together
with no space left between them. Geologist like
simple names, so we call these kind of rocks
“crystalline rocks”. In everyday life, most
people call crystalline rocks “granite”,
but to a geologist, granite is only one specific
kind of crystalline rock.
Some volcanic rocks are hard to break, because
they are solid glass. When lava freezes before
crystals can grow, it makes a natural glass;
obsidian is just one kind of lava-glass.
You might think glass is weak, but that is because
most glass (like windows) is very thin. Natural
glass is not as hard as crystalline rocks though.
Some volcanic rocks are very weak, because they
have lots of bubbles. Lava always has some gas
dissolved in it. As it cools, the gas comes out of
the lava to form bubbles. If the lava freezes
before the bubbles can reach the surface, they get
trapped in the rock. Just like you might expect, a
rock with lots of holes in it is weaker than a
Sedimentary rocks are rocks that are made
when pieces of gravel, sand, or mud get compacted
and glued together by other minerals growing
between them. We call the minerals that grow to
hold all of the pieces together “cement”.
How hard a sedimentary rock is depends on three
1) How much space is there between all of
the pieces of gravel, sand, and mud? If there
is a lot of space, then the rock will be weaker,
just like a volcanic rock with lots of bubbles. If
there isn’t much space then the rock will be
2) What is the cement made of? If
the gravel, sand, and mud are held together by a
mineral like quartz, the rock will be stronger
than if they are held together by a weaker mineral
like gypsum or halite (table salt).
3) What is the sediment made of?
Sandstone (a rock made by cementing sand together)
is usually stronger than mudstone (a rock
made by cementing mud together), because sand is
mostly made of quartz (strong and pretty hard),
while mud is made of minerals similar to talc
(“clays” which are pretty weak and soft).