Cubic System

(Also known as the isometric system) All three axes are of equal length and intersect at right angles. Based on a square inner structure

Crystal shapes include:

• Cube (diamond, fluorite, pyrite)
• Octahedron (diamond, fluorite, magnetite)
• Rhombic dodecahedron (garnet, lapis lazuli rarely crystallises)
• Icosi-tetrahedron (pyrite, sphalerite)
• Hexacisochedron (pyrite).

Common Cubic Crystals:

Tetragonal System

Two axes are of equal length and are in the same plane, the main axis is either longer or shorter, and all three intersect at right angles.
Based on a rectangular inner structure

Crystal shapes include:

• Four-sided prisms and pyramids
• Trapezohedrons
• Eight-sided and double pyramids
• Icosi-tetrahedron (pyrite, sphalerite)
• Hexacisochedron (pyrite).

Common Tetragonal Crystals:

Hexagonal System

Three out of the four axes are in one plane, of the same length, and intersect each other at angles of 60?. The fourth axis is of a different length and intersects the others at right angles.
Based on a hexagonal (6-sided) inner structure

Crystal shapes include:

• Four-sided prisms and pyramids
• Twelve-sided pyramids
• Double pyramids

Common Hexagonal Crystals:

Trigonal System

(Rhombohedral System) - Axes and angles in this system are similar to the Hexagonal System, and the two systems are often combined as Hexagonal. In the cross-section of a Hexagonal crystal, there will be six sides. In the cross-section of a Trigonal crystal there will be three sides.
Based on a triangular inner structure

Crystal shapes include:

• Three-sided prisms or pyramids
• Rhombohedra
• Scalenohedra

Common Trigonal Crystals:

Orthorhombic System

(Rhombic System)Three axes, all of different lengths, are at right angles to each other.
Based on a rhombic (diamond-shaped) inner structure

Crystal shapes include:

• Pinacoids
• Rhombic prisms
• Pyramids
• Double pyramids

Common Orthorhombic Crystals:

Monoclinic System

There are three axes, each of different lengths. Two are at right angles to each other and the third is inclined.
Based on a parallelogram inner structure

Crystal shapes include:

• Basal pinacoids and prisms with inclined end faces

Common Monoclinic Crystals:

Triclinic System

All three axes are of different lengths and inclined towards each other.
Based on a 'triclinic' inner structure, meaning 'three inclined angles'

Crystal forms are usually paired faces

Common Triclinic Crystals:

Amorphous

No crystal structure. Most of these are either cooled too quickly to crystallise, (such as obsidian or moldavite), or are organic (such as amber).

Common Amorphous Minerals:

Author: Janelle Scialla

Our crystal information section contains detailed information about crystlas, their uses and answers to frequently asked questions.

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