Where does the color come from?

What attracts us most about gemstones is of course their beautiful colors. Where do the colors come from? Daylight, often called white light, is actually a mixture of different colors of light. When light passes through a material, the atoms and molecules in the material can absorb some of the light, while the rest passes through. The part that is not absorbed reaches the eye as white light minus the absorbed colors. A ruby appears red because it absorbs all the other colors of white light - blue, yellow, green, etc. - except red.

Some examples of gemstones:

Emeralds are green or yellowish-green and consist of the mineral Beryl - beryllium aluminum silicate - with the known chemical composition of Be₃Al₂(SiO₃)₆:Cr. The crystals are thus built of beryllium, aluminum, silicon and oxygen, and are hexagonal in structure. The green color comes from trace amounts of chromium and/or vanadium.

Rubies are yellow-red or deep carmine red, and their octahedral crystals are built of aluminum oxide, with trace amounts of chromium. The chemical formula is Al₂O₃::Cr

Sapphires are available in virtually all colors but red (red sapphires are known as rubies). Like rubies, the crystals are built of Al2O3, and the color is due to the presence of trace amounts of metals such as iron, chromium, and titanium.

Diamonds are the crystal form of pure elemental carbon. They have no color if there are no impurities present.