What Are the Seven Categories Into Which Crystals Are Classified?

Cubic

• A cube is one of the simplest, most common forms of crystals. All angles are 90 degrees and all three sides are equal in length. Examples include diamonds and garnets.

Triclinic

• There are no 90-degree angles in this crystal structure and no sides are the same length. Examples include turquoise and copper sulfate.

Monoclinic

• Monoclinic crystals have a parallelogram for a base and two sides meet at a 90-degree angle. Examples include azurite and feldspar.

Orthorhombic

• Similar to a cubic crystal, but all three sides are of different lengths. All angles are 90 degrees. Examples include tanzanite and topaz.

Tetragonal

• Similar to a cubic crystal but only two sides are the same length. All angles are 90 degrees. Examples include zircon and wulfenite.

Hexagonal

• These crystals have six sides with three sides of equal length meeting at 60 degrees to each other. Symmetry is six-fold versus the three-fold symmetry of trigonal crystals. Examples include emerald and aquamarine.

Trigonal

• These crystals also have six sides like the hexagonal crystals. However, symmetry is only three-fold as in a three-sided dipyramid or rhombohedron. Examples include amethyst, citrine, ruby and sapphire.