Common Sense Gemstone Grading - by Jeff R. Graham copyright 2008 - Tourmaline

"C" axis's, close, open, dark, light, medium... What?

OK, as we all know the "c" axis on some rough, in particular Tourmaline is not only important, but it is some thing that seems to be difficult for some people to grasp the idea that there are all types of "c" axis's. Because of the varied types there has to be many descriptions.

What do I mean? Well I have run across a few people that insist a "c" axis is either completely open and/or that all the rest is completely closed.

This kind of thinking is like saying there is only one color blue, or there is only one way to do anything. We all know that there are literally hundreds of shades of blue. Any one with a little common sense and experience will also realize that there is almost always more than one way to do some thing.

The fact of the matter is that there are as many kinds of "c" axis's in both saturation and colors as there are people that like to cut Tourmaline. This discussion mostly applies to Tourmaline, but there are a few other types of stones that do have "c" axis's that can be described this way.

The "c" axis's on Tourmaline in particular, can be almost any color and all types of saturations. From totally open to totally closed and every where in between, as well as some pretty interesting color combinations between the "c" axis and the a/b axis's.

So the question is... How do we as faceters describe the "c" axis of a piece of Tourmaline?

The answer is that we figure out a common sense way to describe the stone in question so that some one reading the description has a good idea of what the rough in question looks like.

The most logical and practical way to do this is by using definitions. Before getting to the definitions there are a few other properties of gem materials that need to be mentioned. Tourmaline in particular, but this does apply to other materials as well.

The first thing that should be noted is that with all crystals, the longer the length of the material (crystal) you are looking through the darker and often better (or darker) the color will appear. In some cases of course the darker the color or more saturated the color looks is very miss leading, because often the crystal has to be cut along the lighter a/b axis to either get the desired color or shorten the length of the stone to make it cuttable (or orientate and cut the stone smaller on the "c"). This is simple physics and probably obvious to most people, but I thought it worth mentioning.

How does this effect things? Well in Tourmaline especially if you look through a "c" axis that is for example 20mm in length, the "c" axis may appear very dark or even closed. If you saw the piece in half and look through 10mm in length, the "c" axis may very well have good color and be open. It may not be open, it depends on the material and often there is no way to know with out cutting the piece and looking.

Note: If you can see color looking through the "c" axis of a piece of Tourmaline even a little color, then in almost all cases if the material is cut down the "c" will open up at least a little, some times dramatically.

So what I am saying here is the light/color that is coming through the "c" axis of many pieces of Tourmaline is a variable of the length of the crystal that the light is traveling though. So the length of the crystal is important.

Another thing worth noting is that on some Tourmaline the "c" axis is closed, no light will pass through the "c" axis at all. No matter what length the crystal is. This is a particular type of crystal and only really common in Tourmaline. Usually greens and blues are the colors of Tourmaline most likely to have dark or closed "c" axis's of this type.

One last thing before I get to definitions. Some times the "c" axis in Tourmaline is the same color as the a/b axis's but in most cases the "c" axis (if open) will be at least a little darker color and often a slightly different color, some times completely a different color. For example a typical crystal maybe light green on the a/b and a medium yellow/green on the "c".

It is actually fairly rare for a Tourmaline to be the same color and saturation on ALL axis's. It does happen but is not common. That being said commonly in Tourmaline the "c" axis will be a slightly different color than the a/b axis, but often related in color. For example the a/b maybe a blue/green color and the "c" will be a Kelly green color and usually a bit darker.

One last important note... In Tourmaline and a few other types, but mostly Tourmaline it is possible for the a/b and the "c" axis to be completely different colors. In some cases the colors will not mix together well. For example pink/red and green make a pretty ugly brown. It is not a color combination that is desirable.

Note: There are some unique ways a crystal like this can be cut and make a very interesting stone, but they must be handled differently.

In a case where the colors will mix to make muddy colors, even though the axis is open, I will grade these stones as open because they are... but might note that they need a "closed" axis type of design when cutting them. Basically because when cutting these type of stones they need to be treated like a close "c" axis so that the colors do not mix.

My Definitions:

By my definition a "c" axis is open if light goes through it... Now that does not mean the "c" axis in question is bright and light, it just means that some light will pass. Enough light passes that it is observable. So by this definition we have to have a description to go with the "open" to give a good description of the what the"c" axis truly looks like. Other than some light going through the axis.

Open/Open - This is where the "c" is open and the same saturation or close to the same saturation (note I did not say the same color) as the a/b axis. Both the "c" and the a/b axis in this case are open and transmit light about equally. In this case I often use a color description (modifier) to describe the rough. For example, open/open nice mint green, open/open hot pink, or even some thing like medium green/medium blue/green.

Note: Open/Open means just that, it is describing the axis's as being open and about the same saturations. It is not describing the saturation(s) which maybe light, medium, or dark. So an open/open grade on a stone will need a saturation grade also. Such as open/open medium saturation or what ever the saturation of the stone being described is.

Open/Light - Open and light means that the "c" axis is light in saturation but in general going to be a bit darker than the a/b is. Usually because the length of the "c" axis is longer.

Open/Medium - Open and medium means that the "c" axis is medium in saturation and in general darker than the a/b is.

Open/Dark - Open and dark means that the axis is dark, a bit of light will go through the "c" but really in most cases this type stone will probably require a design that takes into account the dark saturation of "c" axis.

Closed - This means closed, no discernable light is transmitting through the "c" of the stone and this will require a design that is made for dark or closed "c" axis material.

Color description modifier - I will often use a color modifier in the description.
For example...
Green Tourmaline - Open/medium "c" with teal cast. This means the Tourmaline is green a/b and the "c" axis has a medium teal color. The "c" is medium saturation but does transmit some light, not a lot though.

This is just a basic way of describing rough, there are other people with other systems, but I think in most cases if people read through my system it is easy to understand and basically common sense.

This is my system and how I describe my rough.

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