Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: A certain mineral may form in different colors. Figure below shows four samples of quartz, including one that is colorless and one that is purple. The purple color comes from a tiny amount of iron. The iron in quartz is a chemical impurity. Iron is not normally found in quartz. Many minerals are colored by chemical impurities. Other factors can also affect a mineral's color. Weathering changes the surface of a mineral. Because color alone is unreliable, geologists rarely identify a mineral just on its color. To identify most minerals, they use several properties. Answer: iron
Where does the purple color in quartz come from?