Book 27, Question 43
Why do some crystals have stripes or bands?
As a crystal grows layer by layer, conditions around it may change: the temperature, the amount of dissolved minerals, or tiny impurities such as iron.
Each change can leave a colored band. These are called growth bands, a bit like tree rings. Agate, a type of quartz, shows beautiful curved stripes inside, and some amethyst crystals have color zones.