Your question is a good geology question.
Glass is amorphous, meaning that it's atoms are
not arranged in an orderly manner. Minerals, on
the other hand, form in an organized way and their
atoms are lined up in flat sheets or in cubes, for
example. This pattern repeats itself over and over
as the crystal grows.
Glass is sort of like a
mineral in that it is hard and can occur naturally
(like volcanic glass) but glass is not
(doesn't grow in crystals made up of atoms with a
In a mineral crystal,
some atoms are bonded more strongly than others.
Where the bonds are weak, the crystal is most
likely to fracture if you smash it. Take
halite (salt), for example. It's weakest planes
are all 90 degrees from each other so that the
mineral breaks into perfect cubes. The cubes may
be different sizes but they always have the same
You can test me. Use a magnifying
glass and take a look at some salt from your salt
shaker at home. You'll see that all or most of the
salt pieces are little, tiny cubes. (They may not
be perfect cubes anymore if they rubbed on each
other too much, but they'll be close). Because
glass is not crystalline, it breaks into many
different shaped pieces. I hope this helps!