C. Evaluate the Coloring

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.