will find a study of the crystal systems. Most United States
schools teach that there are six crystal systems. In other parts
of the world there are seven. The reason being is that the trigonal
system is very close to the hexagonal system with the exception
that the crystal surfaces are rounded somewhat to give a triagonal
shape rather than a clean, well defined hexagonal or six sided
shape. After much study I believe the British are correct that
the trigonal system is a system all to its own and that there
should be seven systems. But......if you are in gemological school
in the US better give the answer that will pass the test. But
here they all for you to decide...
Please Note: There
are a lot of photo's here so it may be a bit slow to load up.
But the pictures tell everything when it comes to crystal study.
This system is marked
by beautifully formed cubes, octahedra (8 sides) , and dodecahedra
(12 sides). Most notable of the gemstones to form in this system
are diamonds, garnets, fluorite, pyrite, and spinel. Gemstones
forming in the cubic system will be isotropic.
To the left you see
a classic fluorite octahedron. This is the shape that most gem
quality diamonds and spinel will have when they take them from
the ground. Remember, the shapes you are about to see are all
natural crystal shapes and have not been cut or faceted.
Here is a cube of pyrite,
sometimes called fools gold. Pyrite is known for its perfect
This is a big garnet crystal. In fact is weighs over 3.5 pounds or about 7
kilos. If you rotate the stone you will find 12 distinct sides
making it a dodecahedron. It is from a field in Georgia, USA,
where it was found and used for a door stop by a local farmer
for almost 50 years before I obtained it.
The tetragonal system is most often
demonstrated by zircon as shown above. You will note a long rectangular
shape of equal sides and angles. And looking down the "C"
axis or top of the stone (photo #3) you will see the perfect
rectangle shape of the tetragonal crystal. On your examination
for the FGA this will most likely be one of the crystals you
will see. Gemstones forming in the tetragonal system will be uniaxial in optic character.
Most notable about the hexagonal system
is the six sided crystal, as shown at left by these corundum
crystals. Rubies, sapphires, emeralds, and many other gemstones
will form in the hexagonal system. Gemstones forming in the hexagonal
system will be uniaxial in optic character.
As you will
note in the photographs at left, this system is marked by its
almost being a hexagonal shape...but not quite. As noted in the
main photo, the crystals will have a three sided termination.
But as you look down the long or "C" axis of the crystal
you will note that there are three distinct sides, that just
barely fold over into a quasi-six sided crystal. This is where
the debate lies regarding whether or not the trigonal system
is a system into itself or simply a sub-category of the hexagonal
crystal system. In the United States it is taught that the trigonal
system is a sub-system of the hexagonal. However, the GIA does
not go into the study of crystal systems very far in their Graduate
Gemologist courses. Which leaves Graduate Gemologists at a bit
of a loss when confronted with buying trips requiring this specific
expertise. And anyone who has ever compared a nice tourmaline
crystal as shown in the two photographs above, to a green epidote
crystal shown elsewhere in this site, you know that the colors
can be very much alike. But one look at the rough crystals will
make an immediate separation. This is why the study of crystals
is so important to gemologists.
Gemstones forming in the trigonal system
will be uniaxial in optic character. Most notable, and most likely
to show up on your FGA Practical Exam will be a tourmaline...but
study all of them. You never know......
This system is noted for its long prismatic
crystals that taper off on the top to form a cathedral type end.
The crystals themselves are rectangles that look like they have
been smashed down as noted in the photo at left. The is a very
big crystal system containing gemstones such as topaz, peridot,
tanzanite, and many others. Gemstones forming in the orthorhombic
system will be biaxial in optic character.
This system is one of the least symmetrical
of the crystal systems, Until now most systems had some form
or symmetry, meaning as you turn the stone you would see the
same shape repeat itself as the crystal turns. Not so with the
monoclinic system. This is marked by a triangle with no equal
sides or angles as one possible shape (see photo at left) Gemstones
that form in the monoclinic system will be biaxial in optic character. Monoclinic gemstones include kunzite as seen
in this photograph.
system is the least symmetrical of all. Containing no equal sides
or angles, gemstones in this system generally form tabular crystals
as seen here with this amazonite crystal, a type of feldspar.
Gemstones in the triclinic system will have biaxial optic characters.
Additional Crystal Shapes
The above crystal shapes are the classic forms of these systems.
But there are many other shapes that these forms can take. Here
are a few for you to study. There are many more out there to
This is the crystal form most often
seen by sapphire with two pointed ends creating a double pyramid
shape. This is a form of hexagonal crystal growth.
This form presents a jumbled intergrowth
of long spiny crystals as in the case of this growth of calcite
This is a classic form taken by calcite
that is sometimes Iceland spar. It is marked by
a shape looking like a rectangular box that someone pushed from
one side to make it lean.
Here is another look at a cluster of
fluorite cubes. The octahedra fluorite that you see has been
cleaved from these original cube shapes.
This is simply a form where several
crystals of the same species have grown together during the formation
period. You will see specimens of many growing together, or you
may see where only two specimens have grown together as in the
pyrite crystals below.
As noted above this is an example of
two pyrite crystals growing in the same place at the same time.
This kyanite grows in the form of a
blade, sometimes reaching several 30 centimeters or more. This
particular specimen is an example of blades forming in the triclinic
This specimen of malachite form with
this rounded nodule looking form, which will explain the look
of a finished piece of malachite in jewelry. This particular
specimen is an example of botryoidal formation in the monoclinic
Several minerals will form in a tabular
shape. Here we see muscovite from the monoclinic system. Of note
is that many years ago people would find large pieces of this
minerals and separate it along the plate lines and use it for
window panes...until glass windows came along. Its true!
As in the case of opal some gemstones
form without crystal structure. Here you see the conchoidial
breaking of opal that is common with non-crystalline gem materials.
This conglomerate of pyrite crystals
is not exactly cryptocrystalline, but it give you the feel of
the formation. Cryptocrystalline minerals will be made of tiny,
tine crystals forming a larger mineral or rock that shows none
of its crystal parts. An example is carnelian, the orange/brown
member of the quartz family.
Lapis Lazuli is actually a rock. A combination
of several minerals, most notably lazurite, to form a rock. It
is one of only a few rocks used in gem and jewelry making.
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