Do you have pictures or is there any source to visualize tourmaline crystals with different c-axis, light pass through to be seen what an open c-axis look like when it is in rough? I am going to shopping obviously when all subjects finished and I do not want to make mistake. Please help me. Thanks
It isn’t very difficult to detect open vs closed C axis in tourmaline. Usually the color is deeper down the C axis, but not black. Closed C axis is usually only seen in greens and it isn’t a big problem if the color is good on the A-B axis, although, of course, an open C axis is preferred. If it is a good green, without a lot of muddiness, olive green tones, or too light…then it can be cut on the A-B axis and you will get the color you see looking at it on one of these axes. Usually you try to cut the stone in a rectangular step cut to keep the color from mixing with the black coming from the C axis. this is easily done in most of these stones because they tend to be long and thin. You just cut the facets on the end at 65 degrees or more and there just isn’t any light return from the ends and that solves the problem more or less. If the stone is irregular and has an oval or round shape, you wouldn’t want it, because if you cut it into an oval or round, the colors will inevitably mix and the stone won’t look good. You also don’t want very short crystal, as you won’t have much light return on the length of the stone.
So just hold up the stone and look through the ends to see if it is dark (black) or lighter. The color you see will be more or less what you get, maybe a little darker.
The more challenging thing about tourmaline rough is finding pieces which can be cut along the C axis. Pinks and reds tend to be more vivid on the C, but often that means a smaller stone and one with lower yield. There are some greens which are a wonderful blue green on the C axis…this is a very desirable color, but the stone is a bit smaller cut this way, often. As far as greens, there are a lot of them that are a very nice color along the A-B axes.
To add to the above, examine stones, if possible, under incandescent or warm white CFL, as some stones are vivid pink or red in daylight (or daylight LED) but become a bit muddy or brownish under incandescent light. These may still be good stones, just not as good as those which do not get muddy under warm spectrum light. An old style flashlight with a warm toned bulb is in my kit for stone selection. Use your LED flashlight and compare to the color under the old style light…I shouldn’t give away all these secrets…LOL