You may have wondered exactly what is it that makes gemstones so brightly colored, and if you were curious enough to actually pursue that question, you found out that it’s all chemistry. This picture explains it:
Most minerals are actually colorless in their pure form, and they are colored by impurities. The color itself is caused by the different absorption of different wavelengths. Minerals absorb some wavelengths and reject others – the ones which are rejected are the color we see. For example, an object that absorbs all wavelengths of visible light that pass through it, but does not absorb red light, will appear red.
For a more thorough explanation on the phenomenon, see the original source of the picture.