On the net again looking for something interesting in tourmaline. I find pictures of beautiful tourmaline and information that is accurate. Of course, following the lead of our news services, that doesn’t make interesting news. But when I find an embarrassing blunder, well you should know about it. And besides I can get out some of my dismay in the miss represented and miss under stood world of color in tourmaline.
Now the picture on my flat monitor is beautiful. It is the home page of a mineral/gem department of a very large and important American museum (That should know better.) and has as its first picture a beautiful cluster of BI-COLOR tourmaline crystals. You are viewing the crystals basically from the side (A/B axis) and the gemmy crystals clearly show both green and pink color. Wow, what an honor for tourmaline to be featured on such an important page!!! Then I read the caption and the tourmaline cluster is called watermelon tourmaline !!!!???.
Now I love both bi-color tourmaline and watermelon tourmaline and the colors can be identical in both. The classic pink/colorless/green and other more exotic combinations. Similar gemstone can be faceted from both bi-color and watermelon tourmaline (but most watermelon tourmaline are both unsuitable for faceting and lack the size needed to make a decent sized pseudo-bi- color gemstone.) This comes from the DISTRIBUTION of the colors in the crystal and how tourmaline crystals grow in the residual melt from large intrusive bodies of magma. As the pencil shaped tourmaline crystals thicken and change color from a pink center threw a pale zone to a green rind, a watermelon tourmaline is formed. When the pencil shaped tourmaline crystal grows longer and goes from a pink/green zone into the other color a bi-color tourmaline is formed. Because tourmaline is birefringent and therefore dichroic in most cases, the separation of cut stones that are bi-colors and a watermelon tourmaline cut like a bi-color is generally easy. The c axis (down the long pencil shaped tourmaline) is always darker (higher tone level) than the a/b axis across the pencil. (The a axis and b axis must always have the same color/level of saturation because of the crystal system the tourmaline is in). This would result in having darker ends with the bi-color gem and no increase in tone level in the watermelon zoned gem. (Assuming that the gems are oriented with the a/b/c axis when cut. This is very much the “normal” way to cut them.)
Why am I being careful not to make unqualified statements about the orientation and its effect on tone value, etc. when dealing with rough tourmaline. Whether it has a watermelon and bi-color color distribution or not? Because I found a ringer years ago. I think that the rough came from Mozambique and it was deeply water worn. I looked rather like a “finger” with arthritic joints. It was also about half the size of my little finger. There was some yellowish/orange color zoning in the finger, but it was so long and water worn that it was difficult to see down the long axis of the “finger”. While flaws were not a problem, the crystal definitely had to be sliced to efficiently cut the rough. When I finished the slicing, I expected to see the darker c axis as I view the pieces down the original “finger’s” long axis. Well what did I see, but a much lighter a/b axis. This meant that my “finger” had come from a really large tourmaline crystal that had fractured, yet still stayed together while being water worn for million of years, in a most unusual way. (The stones cut were nice, but they lost their exceptional nature when their rough was sliced).
So an exceptional piece of rough helps keep me cautious when talking about the distribution of color/axis in tourmaline rough. That goes for color/hue in tourmaline because there are still exceptional tourmaline out there. The cluster of tourmaline crystals pictured on the Very Big Museum’s home page for the gem and mineral section are obviously bi-colors, not watermelon tourmaline. What a shame that so many well meaning people and organizations talk about tourmaline without adequate knowledge.
Bruce
