Below you will find a study of 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 are all for you to decide…
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 fool’s gold. Pyrite is known for its perfect cube formation.
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 in 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.
This 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 learn about.
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 crystals.
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.
Intergrowth or Twinning
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 system.
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 system.
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 them along the plate lines and use it for window panes…until glass windows came along. It is true!
As in the case of opal some gemstones form without crystal structure. Here you see the conchoidal breaking of opal that is common with non-crystalline gem materials.
This conglomerate of pyrite crystals is not exactly cryptocrystalline, but it gives 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.