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System

Crystals are divided into seven major systems on the basis of:

  • Possible combinations of the elements of symmetry.
  • In terms of three or four imaginary lines of reference called the Crystallographic Axes, which pass through the centre of the crystal.
  • Crystals are allocated to seven systems according to the number of these axes, their lengths and the angles between them.

There is a conventional notation for the lettering and order of the crystallographic axes.

  • In general, the vertical axis is called the "c" axis,
  • That running from right to left is the "b" axis,
  • That running from front to back is the "a"' axis.
  • If the horizontal axes are equal in length, they are referred to as a1 and a2 and the third (vertical) axis is the "c" axis. If all the three axes are equal in length, they are referred to as a1, a2 and a3 and are interchangeable (as in the cubic system).

Crystallographic axes are not the same as the axes of symmetry. For example: In the Hexagonal, Trigonal and Tetragonal systems, the vertical "c" crystallographic axis coincides with the axis of 6-fold, 3-fold and 4-fold symmetry respectively. In the Orthorhombic system all the three crystallographic axes coincide with the three axes of 2-fold symmetry.

The most characteristic crystallographic elements are used in the following description of the seven crystal systems.

Cubic System (Isometric System)  
No. of Crystal Axes: 3
Length: a1 = a2 = a3
Angles: at 90°
Symmetry Elements: a Center, 9 Planes, 13 Axes (6 of 2-fold, 4 of 3-fold, 3 of 4-fold)
Common Forms: Cube, Octahedron, Do-Decahedron, Tetrahedron etc.
Examples: Diamond, Garnet, Spinel, Fluorite, Pyrite, Lazurite etc.

Trigonal System  
No. of Crystal Axes: 4
Length: a1 = a2 = a3 ≠ c
Angles: at 60°, "c" at 90° to their plane
Symmetry Elements: a Center, 3 Planes, 4 Axes (1 of 3-fold, 3 of 2-fold)
Common Forms: Rhombohedron, Prism, Pyramids, Basal Pinacoid etc.
Examples: Calcite, Quartz, Corundum, Tourmaline etc.

Tetragonal System  
No. of Crystal Axes: 3
Length: a1 = a2 ≠ c
Angles: at 90°
Symmetry Elements: a Center, 5 Planes, 5 Axes (1 of 4-fold, 4 of 2-fold)
Common Forms: Tetragonal Prism, Bipyramid, Basal Pinacoid etc.
Examples: Zircon, Idocrase, Rutile, Scapolite etc.

Hexagonal System  
No. of Crystal Axes: 4
Length: a1 = a2 = a3 ≠ c
Angles: at 60°, "c" at 90° to their plane
Symmetry Elements: a Center, 7 Planes, 7 Axes (1 of 6-fold, 6 of 2-fold)
Common Forms: Hexagonal Prism, Pyramids, Basal Pinacoid etc.
Examples: Beryl, Apatite, Benitoite etc.

Orthorhombic System  
No. of Crystal Axes: 3
Length: a ≠ b ≠ c
Angles: at 90°
Symmetry Elements: a Center, 3 Planes, 3 Axes (all of 2-fold, 3 of 2 fold)
Common Forms: Prism, Pyramids, Pinacoids, Dome etc.
Examples: Andalusite, Chrysoberyl, Iolite, Peridot, Topaz etc.

Monoclinic System  
No. of Crystal Axes: 3
Length: a ≠ b ≠ c
Angles: "a" axis inclined to the plane containing "band c", "b and c" at 90°
Symmetry Elements: a Center, 1 Planes, 1 Axes of 2-fold
Common Forms: Prism, Pinacoids, Dome etc.
Examples: Jade, Orthoclase Feldspar, Sphene, Spodumene etc.

Triclinic System  
No. of Crystal Axes: 3
Length: a ≠ b ≠ c
Angles: All inclined
Symmetry Elements: a Center, No Planes, No Axes
Common Forms: Pinacoids, Pedio etc.
Examples: Plagioclase Feldspar, Microcline Feldspar, Kyanite, Turquoise etc.

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In Germany, the garland of ambers is hung around the necks of the infants, so that their teeth might come out without much trouble.

In Turkey, people fix a piece of amber in the hubble-bubble, dogged by an age old belief that the presence of the jewel would destroy all the germs and no disease would spread, even after many have dragged their puffs from it.

In Greece, people think if wine is poured into the cups made of amethyst, one does not get drunk after drinking that.

In Rome, a talisman having coral pieces is considered very auspicious to quell the bad spells of evil look. The Romans were in the habit of hanging chains made of ancient coral pieces from the cradle of the infants to protect them from evil effects.

In China, people wear rings studded with tiny conch or oyster shells as they believe it cures all aches such as stomach ache, worms and like.

In India, pendants made of silver and studded with pearls are hung from the necks of the children to save them from any bad effects.

Greeks still believe, if women wear blue sapphires then no sense of immorality would pollute their mind nor can any fear of the supernatural can trouble them.

The Pope Innocent III had circulated an order asking all the priests wear blue sapphire, so that morality can be strengthened.

It is said about turquoise, that when the stone changes its color into yellow, it actually signals bad times to the person who wears it.

It is believed that a turquoise gifted to a friend or a lover turns his or her life into one of happiness and good luck.

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