We’ve covered how and why crystals grow already, but did you know that there are seven different crystal systems? These crystal systems can have a huge impact on a gemstone’s optical properties.
Confusingly, although there are 14 possible arrangements of atoms, it is simplified down to seven, but there are two different ways of working it out, depending on exactly how you count spacing between atoms. Chemists and gemologists tend to prefer different systems, so we’re going to focus on the system preferred by gemologists, the Crystal Family or Crystal System. These are the shapes formed by the atoms arranged in a crystal and have huge importance when it comes to the optical properties of gemstones. For example, gemstones in the cubic system can never be pleochroic (showing different colours from different angles) since their axes are all the same length – light takes as long to travel in every direction through the gem and so will return to the eye with the same colour no matter which way the gem is turned. It is worth bearing in mind that although a few gems might grow with perfect form, the majority will not, due to defects, irregularities or impurities occurring as they grow.
ISOMETRIC SYSTEM
FEATURES
Also known as the cubic system, this system resembles a cube with three equal perpendicular axes of equal length. Since these lengths are equal, gemstones in this system are all isotropic, meaning that they have the same properties in every direction. Gemstones in this group are all singularly refractive, meaning that they cannot exhibit different colours when viewed from different angles. Well-grown crystals can look like cubes, octahedrons or dodecahedrons.
EXAMPLES
Diamond, Garnet, Spinel, Fluorite, Sodalite, Sphalerite, Pyrite, all precious metals.
TETRAGONAL SYSTEM
FEATURES
The tetragonal system looks very similar to the isometric system, but one axes has been extended, to create a rectangular cuboid. Since the axes differ in length from two directions (known as uniaxialism), gemstones in this system can return light of two different colours to the eye, a phenomenon known as dichroism.
EXAMPLES
Zircon, Rutile, Marialite.
HEXAGONAL SYSTEM
FEATURES
Named after its six-sided form, this structure can either be drawn as a hexagonal prism or as a third of a hexagonal prism. The vertical 'c' axis is longer and at 90° to the shorter axes, meaning that gems in this system are also dichroic. Some chemists would say that there are in fact only six crystal systems and would count hexagonal and trigonal systems as one. However, there is a subtle difference between the two systems.
EXAMPLES
Aquamarine, Emerald, Morganite, Goshenite, Heliodor, Golden Beryl, Apatite.
TRIGONAL SYSTEM
FEATURES
Very similar to the hexagonal system, the trigonal system varies in that the horizontal 'a' axis is not at 90° to the vertical axis. Whereas hexagonal gems have six identical faces, trigonal structures only have three. Trigonal gemstones are also all dichroic.
EXAMPLES
Quartz, Tourmaline, Ruby, Sapphire, Calcite.
ORTHORHOMBIC SYSTEM
FEATURES
The atoms in this system form an off-centred pyramid or a bipyramid (two pyramids base to base), with three axes of unequal length, but intersecting at 90° with the base of the pyramid. Gemstones in this system are trichroic, meaning that due to the three unequal axis lengths, light travels through the gem at three different speeds, producing three different colours.
EXAMPLES
Andalusite, Iolite, Tanzanite, Topaz, Peridot, Chrysoberyl, Csarite, Chrysocolla, Danburite, Dumortierite, Enstatite, Marcasite, Prehnite, Shattuckite.
MONOCLINIC SYSTEM
FEATURES
The monoclinic system again has three axes of unequal lengths, but in this system, only two of them, the vertical and horizontal, meet at 90°. The third angle meets at less than 90°, creating a base that is a parallelogram. The word ‘monoclinic’ comes from the Greek words ‘mono’, meaning ‘single’, and ‘klineien’ meaning ‘slope’, due to the fact that there is only one face that is mirrored. Gemstones in this system are also trichroic.
EXAMPLES
Diopside, Sphene, Kunzite, Moonstone, Clinohumite, Howlite, Jadeite, Lepidolite, Malachite, Orthoclase, Sanidine.
TRICLINIC SYSTEM
FEATURES
Very similar to the monoclinic system, the triclinic system’s only difference is that no angles meet at 90°. All axes and angles are different from one another. It has no symmetry in any direction. Gemstones in this system are also trichroic.
EXAMPLES
Turquoise, Kyanite, Sunstone, Labradorite, Andesine, Lehrite, Serenite, Rhodonite.
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