The color in gems
may not be evenly distributed. This is most evident in stones
like Ametrine and watermelon tourmaline, which are cut to show
off the separate colors. In most gems though, you need to look
carefully to see it.
Amethyst
is known for its "zoning". It may have areas of dark purple
surrounded by lighter purple or even colorless quartz. With careful
cutting, orienting the color in the culet, the gem will be evenly
colored face up. This brings the gem to its highest beauty and eye
appeal. If you can see the zoning face up, or with just slight tipping,
it reduces the value of the gem.
This is
also common in sapphire. The same principle applies, if you can't
see the zoning face up; it has no effect on the value of the gem.
The zones of color in an amethyst will be random, while the zoning
of a sapphire is in straight bands. They are present even if they
aren't distinct enough to be seen with the naked eye. The best way
to see them is to view the gem from the bottom with light coming in
from behind it.
You have
to be looking directly on to the plane of color to distinguish the
banding. Due to the cutting, the banding will only be visible in one
facet at a time. It takes a bit of practice to see this, but it is
worth learning.
When you see the banding, check to see if it is straight or curved.
If it there is any curvature, it will be subtle so you have to look
carefully to perceive it. However, this is very significant. If the
striae, the banding, are curved that means it is a flame fusion synthetic.
Straight banding can be natural or synthetic.
So you
see how much information you can get from a loupe. If you are a beginner,
this will probably bring up dozens of new questions and they will
direct you to your next level of study.