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Gemstone Crystal structures

Crystallography is the study of crystal structures and encompasses the geometrical, physical, and chemical properties of a crystal. The field can be very complex, requiring a mastery of not only geometry, but physics, and chemistry as well. Our purposes here are to give only a very high level introduction to the geometric aspects so that the terms will have some meaning when comparing the crystal form of two different types of gem. There are differing opinions as to how many structures there are but most agree that there are 32 or 33 non-cubic forms, and 15 cubic forms. Experts also disagree on whether there are 6 or 7 basic structures that concern us as gemologists. Most, if not all, structures have derivations from the basic type ie. the 15 types of the cubic structure.

The cubic structure, to most of us, means a six-sided object with all sides having the same dimension and all angles being 90 degrees, as in the first figure. Crystallography recognizes 15 cubic forms ranging from the cube with 6 sides, to the octahedron with 8 sides, the second figure to the hexoctahedron with 48 sides, the last figure. The important thing is that the basic law, three axis of equal length form 90-degree angles between any two axes, still holds true. To form a 6-sided cubic structure, draw three axis and add square planes at the ends of the axis, to form an octahedron, use the same three axis, but add triangular planes with edges and points connected. A hexoctahedron still follows the basic 3-axis rule, but it contains several sets of 3-axis structures

Crystals structures are grouped into three basic structure groups depending on the number of equal dimensions. The first group, trimetric, contains three axis of equal length therefore only includes the cubic structure. The second group or dimetric group, consists of the trigonal, tetragonal and hexagonal structures. This group has two axis of similar length, with a third and possibly a fourth axis of a different length. The third and final group is the monometric group with no two axis the same. This group includes the monoclinic, triclinic and orthorhombic structures.

	Cubic The cubic structure is characterized by three equal axis - a,b and c. This structure provides a cubic shape with all sides sides having the same dimensions.
	Tetragonal The tetragonal structure is characterized by two equal axis - b and c with a being a different length.With all angles at 90 degrees, this structure provides an elongated cubic shape with the end view being square and the side views being rectangular.
	Hexagonal The hexagonal structure is characterized by three equal axis - a, b and c with e being a different length.With all angles between a,b and c at 60 degrees to each other and 90 degrees to e, this structure provides an elongated, or shortned hexagonal form with flatened ends.
	Trigonal The trigonal structure is characterized by three equal axis - a, b and c with e being a different length.With all angles at 60 degrees, this structure provides an elongated or shortned hexagonal shape from any side, and a perfect hexagonal shape from the end.
	Orthorhombic The orthorhombic structure is characterized by no equal axis - all three are a different length.With all angles at 90 degrees, this structure provides a rectangular shaped structure from any direction.
	Monoclinic The monoclinic structure is characterized by no equal axis - all three are a different length.With one angle at 90 degrees, this structure provides a rectangular shaped structure with a wedge shape end.
	Triclinic The triclinic structure is characterized by no equal axis - all three are a different length.With no angle at 90 degrees, this structure provides an irregular shaped structure with no square corners.

Gemstones may exhibit several different derivatives of a form, such as the garnet that often exhibits both the dodecahedron and tetrahexahedron variation of the cubic structure (12 and 24 sides respectively).

In showing the seven types here, we have seriously over-simplified a very technical subject. For a more detailed discussion of crystallography, see the following sites... but be prepared, you may need to hold a doctorate in geometry to understand some of it!

Introduction to Crystallography and Mineral Crystal Systems
by Mike Howard at rockhounds.com

Crystal Systems
at mineral.galleries.com

Introduction to Mineralogy class
by J. Rice at Emory University

Crystallography
at webmineral.com . . . This site shows rotating models of various structures

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