While impurities don t make a significant difference in the chemistry of a mineral, they can make a significant difference in its appearance and this can have a considerable effect on its value. A pure mineral is usually colorless and it is the impurities that give it color. An excellent example is corundum. In its pure form it is completely colorless. Add a bit of chromium and we call it a ruby, a bit of titanium and iron and it becomes a blue sapphire.
Pure beryl is also colorless.
Add a touch of chromium and you have an emerald, a bit of iron and you get an aquamarine. Corundum and beryl are called mineral species. Their colored versions are varieties. Another very common species is quartz, which has the varieties of amethyst, citrine and smoky quartz. Now there always has to be an exception or two, so here we go.