What are Crystal Systems and Mineral Habits?


Step 1: Introduction to Gemology

Lesson 7

Sulfur - crystal systems
When sulfur forms into crystals, it forms in the orthorhombic system, just like alexandrite and topaz. There are six crystal systems or ways that crystals form within specific minerals. “Sulfur,” Cozzodisi Mine, Agrigento Province, Sicily, Italy. © Rob Lavinsky, www.iRocks.com. Used with permission.

In crystallography, mineral habits refer to the way crystals form within a specific mineral. In the article, “How Gems Are Classified,” I mentioned that, at the molecular level, diamond looks like two pyramids attached at their bases and quartz has six sides. These are examples of “mineral habits” or “crystal systems.”

How Are Crystal Systems Defined?

There are six crystal systems. All minerals form crystals in one of these six systems. Although you may have seen more than six shapes of crystals, they’re all variations of one of these six habits. Each system is defined by a combination of three factors:

  • How many axes it has.
  • The lengths of the axes.
  • The angles at which the axes meet.

An axis is a direction between the sides. The shortest one is A. The longest is C. There is a B axis as well and sometimes a D axis.

The Isometric System

The first and simplest crystal system is the isometric or cubic system. It has three axes, all of which are the same length. The three axes in the isometric system all intersect at 90º to each other. Because of the equality of the axes, minerals in the cubic system are singly refractive or isotropic.

Isometric – crystal systems
The isometric crystal system has three axes of the same length that intersect at 90º angles.

Minerals that form in the isometric system include all garnets, diamond, fluorite, gold, lapis lazuli, pyrite, silver, sodalite, sphalerite, and spinel.

Isometric shapes – crystal systems
Minerals that form in the isometric system form in one of these three basic shapes.

The Tetragonal System

The tetragonal system also has three axes that all meet at 90º. It differs from the isometric system in that the C axis is longer than the A and B axes, which are the same length.

Tetragonal – crystal systems
The tetragonal crystal system also has three axes. Axis C is longer than axes A and B, which are the same length.

Minerals that form in the tetragonal system include apophyllite, idocrase, rutile, scapolite, wulfenite, and zircon.

Tetragonal shapes – crystal systems
Minerals that form in the tetragonal system form in one of these three basic shapes.

The Orthorhombic System

In this system there are three axes, all of which meet at 90º to each other. However, all the axes are different lengths.

Orthorhombic – crystal systems
The orthorhombic system has three axes, each of which is a different length. These axes intersect at 90º angles.

Minerals that form in the orthorhombic system include andalusite, celestite, chrysoberyl (including alexandrite), cordierite, iolite, danburite, zoisite, tanzanite, thulite, enstatite, hemimorphite, fibrolite/sillimanite, hypersthene, olivine, peridot, sulfur, and topaz.

Orthorhombic shapes - crystal systems
Minerals that form in the orthorhombic system form in one of these three basic shapes.

The Monoclinic System

The previously discussed crystal systems all have axes/sides that meet at 90º. In the monoclinic system, two of the axes, A and C, meet at 90º, but axis B does not. All axes in the monoclinic system are different lengths.

Monoclinic – crystal systems
The axes in the monoclinic system are all different lengths. The A and C axes intersect at 90º, but axis B does not.

Minerals that form in the monoclinic system include azurite, brazilianite, crocoite, datolite, diopside, jadeite, lazulite, malachite, orthoclase feldspars (including albite moonstone), staurolite, sphene, and spodumene (including hiddenite and kunzite).

Monoclinic shapes – crystal systems
Gems that form in the monoclinic system form in one of these three basic shapes.

The Triclinic System

In the triclinic system, all the axes are different lengths. None of them meet at 90º.

Triclinic – crystal systems
None of the axes in the triclinic system intersect at 90º and all are different lengths.

Minerals that form in the triclinic system include amblygonite, axinite, kyanite, microcline feldspar (including amazonite and aventurine), plagioclase feldspars (including labradorite), rhodonite, and turquoise.

Triclinic shapes – crystal systems
Gems that form in the triclinic system form in one of these three basic shapes.

The Hexagonal System

The crystal systems previously discussed represent every variation of four-sided figures with three axes. In the hexagonal system, we have an additional axis, which gives the crystals six sides. Three of these are equal in length and meet at 60º to each other. The C or vertical axis is at 90º to the shorter axes.

Hexagonal – crystal systems
The hexagonal system has four axes. Three are equal in length and intersect at 60º. The longer C or vertical axis intersects the other shorter axes at 90º.

Minerals that form in the hexagonal system include apatite, beryl (including aquamarine, emerald, heliodor, and morganite), taaffeite, and zincite.

Hexagonal shapes – crystal systems
Gems that form in the hexagonal system form in one of these two basic shapes.

The Trigonal Subsystem

Mineralogists sometimes divide the hexagonal system into two crystal systems, the hexagonal and the trigonal, based on their external appearance. (Corundum, both ruby and sapphire, is sometimes described as trigonal). However, for gemological purposes, the above six categories are sufficient.

Trigonal Shapes - crystal systems
Most gem guides will list trigonal crystals as hexagonal. These crystals are sometimes distinguished from hexagonal crystals because of their appearance.

Gem Material Without Crystal Systems

Amorphous materials are not minerals. Thus, they don’t form in any of these crystal systems. Examples of amorphous materials used as gems include amber, glass (including obsidian), ivory, jet, moldavite, and opal.

Some materials used as gems may contain crystals of minerals but can’t themselves be described as crystals because they don’t have a uniform crystal structure. These materials are called polycrystalline.

Agate - crystal systems
Agate is a variety of quartz (which has a hexagonal mineral habit) that is an aggregate or polycrystalline material. “Chalcedony: (Var. Agate),” Juchem Quarry, Niederworresbach, Idar-Oberstein, Hunsruck Mts, Rhineland-Palatinate, Germany. © Rob Lavinsky, www.iRocks.com. Used with permission.

About the author
Donald Clark, CSM IMG
Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters "CSM" after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff's ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book "Modern Faceting, the Easy Way."
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