How Trace Elements Color Corundum
Corundum—the mineral that forms rubies and sapphires—is aluminum oxide. In its pure form, it has no visible color. The vivid hues appear when a small number of aluminum atoms in the crystal are replaced by atoms of other elements as the mineral forms.
Those substitutions act like selective filters. Light contains many wavelengths, which we perceive as different colors. Trace elements absorb some wavelengths inside the stone; the light that is not absorbed can pass through or return to the eye, giving the gem its apparent color.
Why Chromium Makes Ruby Red
Chromium absorbs portions of the light spectrum while allowing red wavelengths to remain prominent. The result is red corundum, known as ruby. The same basic crystal structure can look entirely different when iron and titanium are present together: their interaction helps absorb light in a way that makes the remaining light appear blue.
That blue corundum is called sapphire. Other trace elements and combinations can produce other colors, so “sapphire” is not limited to blue stones.
One Mineral, Different Colors
The distinction between ruby and sapphire is a color-based naming distinction, not a difference in the mineral’s fundamental recipe. Small chemical substitutions change how the crystal handles light without turning it into a different mineral. A few trace atoms can transform colorless corundum into a red ruby or a blue sapphire.
