Step 1: Introduction to Gemology

Lesson 7

In crystallography, mineral habits refer to the way crystals form within a specific mineral. In A Guide to Gem Classification, 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.”
By Donald Clark, CSM IMG 4 minute read
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 crystals, Cozzodisi Mine, Agrigento Province, Sicily, Italy. © Rob Lavinsky, www.iRocks.com. Used with permission.

Table of Contents:

  • How are Crystal Systems Defined?
  • The Isometric System
  • The Tetragonal System
  • The Orthorhombic System
  • The Monoclinic System
  • The Triclinic System
  • The Hexagonal System
    • The Trigonal Subsystem
  • Gem Material Without Crystal Systems

How are Crystal Systems Defined?

There are six crystal systems, and 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

Isometric system

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

Isometric shapes

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

Tetragonal system

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

Tetragonal shapes

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

Orthorhombic system

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

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

Monoclinic system

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

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

Triclinic system

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

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

Hexagonal system

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

Hexagonal shapes

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

Trigonal crystals are sometimes distinguished from hexagonal crystals because of their appearance.

Gem Material Without Crystal Systems

Amorphous materials aren’t minerals because 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

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