From The Gemology Project
Crystal Systems & Forms
| Crystal System
The cube is composed of 6 square faces
at 90° angles to each other. Each
face intersects one of the crystallographic
axes and is parallel to the other two.
|| Diamond |
The tetragonal system also has three axes
that all meet at 90°. It differs from the
isometric system in that the C axis is longer
than the A and B axis, which are the same length.
All meet at 90°.
In the hexagonal system, we have an additional axis,
giving the crystals six sides. Three of these
are equal in length and meet at 60° to each other.
The C or vertical axis is at 90° to the shorter axes.
Mineralogists sometimes divide this into two systems,
the hexagonal and the trigonal, based on their external
appearance (see following).
Again, the trigonal system is a subsystem of the
hexagonal. Most gem references will list these as
In this system, there are three axes all of which
meet at 90° to each other. However, all the axes
are a different length.
The above crystal systems all have axes sides
that meet at 90°. In the monoclinic system, all
the axes are different lengths. Two of them,
the A and C axes, meet at 90°, but the third
one does not.
In this system all the axes are different
lengths and none of them meet at 90°.