How gemstones get their colors

By Yves Lemay Jul 31, 20

How gemstones get their colors

Color in minerals is caused by the absorption or lack of absorption of various wavelengths of light. 

When pure white light, containing all wavelengths of visible light, enters a crystal, some of the wavelengths might be absorbed while other wavelengths may be emitted. 

If this happens, then the light that leaves the crystal will no longer be white but will have some color.

Elements that produce colors through absorption and emittance of wavelengths are usually transition metals. 

They can cause a mineral always to be a certain color if they are part of the mineral's chemistry. 

However, if there is just a trace of these elements, they still can strongly influence the color of the mineral. 

Even tiny amounts of these elements can deeply color minerals. 

It is erroneously thought that certain elements cause only certain colors, and there is some truth to that. 

Copper usually produces green and blue colors. 

Iron is known for the red and yellow colors that it typically provides. 

However, almost any element can be responsible for any color.

Below is a list of some coloring elements and the color they produce in at least one mineral:

• Cobalt, Co, produces the violet-red color in erythrite, (cobalt arsenic sulfide).

• Chromium, Cr, produces the color orange-red color of crocoite, (lead chromate).

• Copper, Cu, produces the azure blue color of azurite, (copper carbonate hydroxide).

• Iron, Fe, produces the red color of limonite, (hydrated iron oxide hydroxide).

• Manganese, Mn, produces the pink color of rhodochrosite (manganese carbonate).

• Nickel, Ni, produces the green color of annabergite, (hydrated nickel arsenate).

• Uranium, U, produces the yellow color of zippeite, (hydrated potassium uranyl sulfate hydroxide).

• Vanadium, V, produces the red-orange color of vanadinite, (lead vanadate chloride).

Although still subject to the effects of trace elements, these minerals always have the same primary color. 

However, most minerals are usually white or colorless in a pure state, unless they are idiochromatic like the peridot.

Many impurities can color these minerals and make their color variable. 

The property of streak often demonstrates the true or inherent color of a mineral. 

In addition to coloring elements, other impurities or factors exist that have also been linked to the color of minerals. 

Such things as elemental fluorine, sulfur, and chlorine; trace amounts of carbonate and other ion groups; chlorine and fluorine ions and even structural defects. 

Radiation from rare earth minerals can damage a crystal structure, and this damage seems linked to coloring as in smoky quartz and green diamonds. 

Care should always be given when trying to identify a mineral using color. 

Even in minerals that have a variable color, a skilled and experienced mineralogist can use the color of these minerals to make a positive identification.

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