It may be surprising that the first thing that most people notice about a mineral, its color, is usually not very reliable for identifying it. That is because many minerals occur in a wide range of colors, colored by slight impurities. We can explore that by using a few things from your kitchen.
You will need:
- several clear drinking glasses
- food coloring
- a spoon for stirring
Fill one glass about half-full of water. Place it on the table and look at it. What color is it? Clear, right? No surprise there. But, what if we added something to the water? Not something that would react with it chemically, to change it into something else. We just want something to change its color.
Adding coloring does not change the chemistry of water. No matter what color, it is still water.
Fill another glass about half-full of water, and place it on the table beside the first. Add a drop of food coloring to the water and give the water a quick stir. Compare it with the original glass of water. It’s a different color, but is it still water? Yes. Adding a tiny amount of coloring did not change its chemical formula. It is still water. Using different colorings, you can have water that is white, purple, black, pink, or any other color. As long as you don't change the water chemically, it is still water, no matter what color it is.
The same is true for the mineral quartz. It occurs in many colors, but they are all quartz.
Now, lets think about minerals in the same way. One of the most common minerals is quartz. Pure quartz is clear, just like pure water. Tiny amounts of impurities can color the quartz just as the food coloring colored the water. In the photo above, the crystal on the left is a pure, clear quartz crystal. But, what if that crystal was colored by impurities?
- Tiny bubbles of gas or liquid inside the mineral can produce white quartz, also known as milky quartz.
- Iron can color the quartz purple. Then, we would call it amethyst.
- Exposure to radiation can free some atoms of silicon from the quartz, coloring it grey or black. In that form, it is known as smoky quartz. This can be because of exposure to natural radiation, but some people change clear quartz crystals to smoky quartz by exposing them to radiation.
- Titanium, iron, or manganese can color quartz pink, which is commonly known as rose quartz.
All these varieties are still quartz. They all have the same chemical formula, the same hardness, the same fracture, the same crystal structure. They are different varieties of the same mineral.
With that said, there are times when color is useful in narrowing down the possibilities. While there are several minerals that can be purple, there are quite a few minerals that have never been found with a purple color. A bright purple specimen might be quartz, fluorite, or one of several other minerals, but it is probably not orthoclase or biotite. They just don't occur in a purple color. In cases where color may help, it will be noted on the identification chart under "other properties."