GO 340 Gemstones & Gemology
Emporia State University 
www.emporia.edu/earthsci/amber/go340/crystal.htm
Crystal Structure
| Minerals have a characteristic crystalline structure. This means they grow in a very regular, symmetrical arrangement. The crystalline or atomic structural arrangement determines optical properties, as well as physical properties, such as external shape, hardness, cleavage or fracture, and specific gravity. Although a crystalline structure is constant for all minerals, not all minerals are perfectly shaped or found with euhedral crystal forms. Many have an irregular external shape or arrangement of crystal faces, termed subhedral or anhedral. When the external shape is not necessarily a perfect reflection of the internal arrangement of atoms or showing its form, it is called the mineral's habit. These terms, form and habit, can be descriptive such as octahedral and prismatic forms or equant and acicular habits. Left and center images illustrate this. The external shape of minerals and gems can be influenced by composition and structure, as defined by pseudomorphism, polymorphism, and isomorphism. | 
Natural diamond
showing an
octahedral form
or equant habit.
Image taken from
The Image Gallery | 
Rutile included in
quartz resembles blonde
hair and has a prismatic form
or an acicular, needle-like
habit. | 
Hawk's-eye rough
and polished. |
means false form or shape. It is when a mineral crystal's chemical composition and/or crystal structure is changed, but the external form is preserved. An example is in the alteration of azurite crystals to malachite, and the replacement of wood by chalcedony to produce petrified wood. A common misconception is that tiger's-eye and hawk's-eye (blue tiger's-eye) are quartz pseudomorphs after the asbestos mineral crocidolite, that is the quartz replaced the asbestos fibers. Hawk's-eye shown in image above and right. This has recently been shown to be a false assumption. Heaney and Fisher (2003) have a new interpretation of the origin of tiger's-eye in that quartz crystal growth is synchronous with the crocidolite through a crack-seal vein-filling process (p. 323).
means many forms or shapes. It is when minerals have the same chemical composition but different crystal structures resulting in different minerals. Diamond and graphite are both composed of carbon but obviously different minerals with vastly different physical and optical properties. Graphite is the soft pencil lead and used as a lubricant, while diamond is a hard gemstone and used as an abrasive for cutting in its industrial applications.
means one or the same form or shape. Isomorphic minerals have the same geometric structural arrangement, but with different atoms or ions in the sites that results in different minerals. Minerals with the same anion belong to isostructural groups, such as garnet and spinel groups. These groups have the same structural configuration but a wide diversity in chemical composition. A detailed explanation of isomorphism using the garnet family of minerals is given by Robert James, Your Gemologist, at http://www.yourgemologist.com/IsomorphousReplacement/isomorphous.html.
Crystal system is a classification scheme developed to explain the orderly atomic arrangement of the chemical components that make up minerals. All minerals can be divided into one of six classifications. These systems are defined by the imaginary crystallographic axes and the angles between the axes. The isometric system has three equal crystallographic axes, all at right angles to one another. Typical crystal shapes include a cube, octahedron, and dodecahedron. The tetragonal system has three axes meeting at right angles, but two are the same length, the third is longer or shorter than the other two. These crystals are four-sided prisms and pyramids. The orthorhombic system has three axes meeting at right angles and none of the lengths are equal. The typical shapes are rhombic prisms and pyramids. The hexagonal system has four crystallographic axes, three in one plane, all the same length, and intersecting each other at 120 degrees. The fourth axis is perpendicular to the plane of the other three and a different length than the other three. Typical crystal shapes are hexagonal prisms and pyramids. The monoclinic system has three axes of different lengths, two at right angles to each other and the third is inclined. Typical forms are pinacoids (paired faces) and prisms. The triclinic system has three axes all of different lengths and all inclined to each other. This system is the least symetrical and shapes are usually pinacoids.
Visit Robert James, Your Gemologist, on crystal systems at http://www.yourgemologist.com/crystalsystems.html now.
- Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.
- Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing.
- Heaney, P. J., & Fisher, D. M. (2003). New interpretation of the origin of tiger's-eye. Geology, 31(4), 323-326.
- Amethyst Galleries, Inc. (1995-1996). The mineral diamond. WWW URL; http://www.galleries.com/minerals/elements/diamond/diamond.htm. Retrieved February 2, 2004.
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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: November 15, 2000; last update: January 14, 2008.
Copyright 1999-2008 Susan Ward Aber. All rights reserved.