Crystals are divided into one of 7 different classifications. Following are a description of each of these systems.
A crystal system where the cell dimensions are equal, and all inter-axis angles are 90 deg. This has the highest form of symmetry possible. It is composed of three crystallographic axes of equal length and at right angles to each other. There are 15 forms, all closed, in the Isometric Crystal system. This property of being devoid of a face is unique to this system.
A crystal system characterised by the presence of a hexad (axis of 6-fold rotation). Two of the axes are 120 deg apart and are of equal length. The third axis (c) is aligned with the hexad, and is 90 deg to the other two axes. The hexagonal system is uniaxial, meaning it is based on one major axis. The hexagonal and trigonal systems are unlike any of the other systems in terms of crystallographic axes. While the other systems use three crystallographic axes, the hexagonal and trigonal systems use four axes.
A crystal system where the cell dimensions (a,b,c) are of unequal length. Two of the axes are at 90 deg to each other, whilst the third is not. The monoclinic system is the largest symmetry system with almost a third of all minerals belonging to one of its three classes.
A crystal system where the cell sizes (a,b,c) have different lengths, but all of the inter-axis angles are 90 deg. The orthorhombic system is based on three unequal axes all at right angles to each other.
A crystal system where two of the axis are of equal length, with the vertical axis being of a different length, either longer or shorter. All inter-axis angles are 90 deg. A requirement of this type of system is the four fold rotational axis.
A crystal system where the three cell dimensions are unequal length, and none of the corresponding axes are at 90 deg. This is the lowest form of crystal symmetry possible and contains only two symmetry classes. One class contains only a center and the otherclass is left with no symmetry at all.
This is a crystal system which is similar to the hexagonal system. This is characterised by a triad (axis of 3-fold rotation). Two of the cell axes are 120 deg apart, and are both 90 deg from the c axis which is aligned with the triad.
The well known Mohs scale was named for Friedrich Mohs (1773-1839), a noted German mineralogist. He developed his system of testing the hardness of crystals in 1812 CE.
It is a scale for classifying minerals based on relative hardness, determined by the ability of harder minerals to scratch softer ones. The scale includes the following minerals, in order from softest to hardest: