How to Describe a Tourmaline Crystal by its C Axis


tourmaline crystal - baguette cut
This darker teal/blue-green gem was cut to suit a stone with a dark C axis. “Tourmaline,” fancy baguette cut, 1.47 cts, Congo. © Dan Stair Custom Gemstones. Used with permission.

We know gem cutters must evaluate the C axis, the longest direction between a crystal’s sides, before cutting a tourmaline crystal. However, I’ve found that some people insist that a C axis is either completely open (allowing light to pass through) or closed (allowing no light to pass through). That’s like saying there’s only one color blue when literally hundreds of shades of blue exist. In fact, there are as many kinds of C axes in terms of both saturation and color as there are people that like to cut tourmaline crystal. Furthermore, the C axis and the A/B axes can make some pretty interesting color combinations. Due to this variety, we need a system to describe the C axis effectively. This way, anyone reading our descriptions will have a good idea of what the rough looks like.

While this article applies mostly to tourmaline, the descriptions I’m proposing can be used for a few other types of gemstones with C axes.

hexagonal system - tourmaline crystal
Tourmalines have a trigonal crystal system, which falls under the hexagonal system. 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.

Tourmaline Crystal Properties That Influence C Axis Descriptions

Before finding a common sense way to grade the C axis of a particular tourmaline piece, let’s discuss some gemstone properties that will impact our evaluations. These apply to tourmaline in particular but also affect other gemstones.

Crystal Length

The longer the length of the crystal material you’re looking through, the darker the color will appear. This applies to all gemstone crystals. Of course, this can be misleading. In some cases, gem cutters may have cut crystals along the lighter A/B axis. For example, they may do this to get the desired color. They may even do this to make the stone cuttable.

Take a tourmaline crystal and look through its C axis. At, say, 20 mm in length, the C axis may appear very dark or even closed. If you saw the piece in half and look through a 10 mm length, what will you see? You may find the C axis has good color and appears open. Or, it may be closed. This depends on the material itself. Often, you can’t know the result without cutting the piece and taking a look. However, in almost all cases, if you can see color, even just a little, when looking through the C axis of a tourmaline crystal, cutting down the C axis will open it up at least a bit (and sometimes quite a bit).

Dark or Closed C Axes

Some tourmalines, usually greens and blues, may have dark or closed C axes no matter the length of the crystal. No light will pass through the C axis.

All Axes May Have the Same Color

In most cases, tourmalines have C axes (if open) with slightly darker colors than the A/B axes. Often, the C axes will show slightly different colors. For example, a tourmaline crystal may show light green on the A/B axes and a medium yellow-green on the C axis. Usually, the colors are related. So, if the A/B axes show a blue-green color, the C axis may be a darker kelly green color.

However, in rare cases, tourmalines may have the same color on all axes. Even more rarely, the axes may share the same saturations, too.

Undesirable Color Combinations

Some tourmalines may have C and A/B axes of completely different colors. Sometimes, these colors don’t mix well. For example, pink/red and green makes a pretty ugly brown, in my opinion. Still, you could cut such a tourmaline crystal in a unique manner and make it an interesting stone. (That’s a topic for another article).

In cases where the colors will mix to make muddy colors, even though the axes are open, I’d describe these stones as open. (After all, they are open). However, I’d note they need a “closed” axis type of design for cutting. Basically, cutters should treat them like closed C axis stones so the colors don’t mix.

C Axis Grades for a Tourmaline Crystal

By my definition, a C axis is open if light goes though it. This does NOT mean an open C axis is bright and light. All it means is that enough light passes to be observed. Therefore, we need to add modifiers to “open” to truly describe what the C axis looks like.

I suggest the following grades.

Open/Open

An open C axis with the same saturation (or close) as the A/B axes. The C and A/B axes are open and transmit light about equally. In this case, I often use additional color modifiers to describe the rough. For example, open/open nice mint green or open/open hot pink.

Please note: this grade doesn’t imply the colors are the same. Nor does it describe the saturations beyond being the same. The saturations may be light, medium, or dark. Therefore, a stone with an open/open grade needs a saturation grade, too.

Open/Light

An open C axis with light saturation but still darker than the A/B axes, usually because the C axis is longer.

Open/Medium

An open C axis with medium saturation but still darker than the A/B axes.

Open/Dark

Although a bit of light goes through the C axis, this grade means the stone requires a design that accounts for the dark saturation of the C axis.

Closed

No discernible light gets through the C axis. Such a stone requires a design made for dark material.

Color Modifiers

I often use color modifiers when grading the C axis of a tourmaline crystal. For example:

Green Tourmaline: Open/Medium, “C” with teal cast.

This means the A/B axes are green and the C axis has a medium teal color. The “C” has a medium saturation but does transmit some light, though not a lot.

A Basic System for Describing Rough

I believe my common sense system is easy to understand. If you read this article, you’ll have a way to describe a tourmaline crystal or other gemstone rough so a cutter can visualize it accurately.

tourmaline crystal - rough
“Tourmalines,” Brazil, by Mauro Cateb. Licensed under CC By 2.0.

About the author
Jeff R. Graham
The late Jeff Graham was a prolific faceter, creator of many original faceting designs, and the author of several highly-regarded instructional faceting books such as Gram Faceting Designs.
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