Color in jewels cannot be distributed evenly. It is very noticeable in such stones as аметрин and a tourmaline which even cut so that to show a color separation. In many jewels, at attentive viewing you can also notice this phenomenon.
In amethyst it is possible to notice "zones" of darkly violet, light-violet or colourless quartz. It does a stone extremely beautiful.
The same is widespread among sapphires. Only color at sapphire blue.
The painted zones in amethyst are distributed in a random way, and in sapphire are united in groups. At first sight and not to see them. The best way to see them - to consider a stone on a gleam.
It is necessary to practise a little to see it, but tutoring is worth it.
You can obtain a lot of information, applying a magnifying lens. If you the beginner, it can probably bring you several tens questions, and they will direct you to your following step of tutoring.