Step 1: Introduction to Gemology

Lesson 7

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…