Ruby
Definition and meaning of Ruby in chemistry.
Ruby is a gemstone composed of aluminum oxide (corundum) with chromium impurities that cause its characteristic red color. Chromium(III) ions substitute for aluminum ions in the crystal lattice and absorb blue-yellow light wavelengths, allowing red light to dominate.
In more detail
In ruby's crystal structure, chromium(III) ions occupy lattice sites normally filled by aluminum. These ions absorb light wavelengths in the 400-600 nm range (blue-yellow visible spectrum) through electronic transitions in their d-orbitals, a phenomenon explained by crystal field theory. This selective light absorption allows red and near-infrared light to pass through or be reflected, creating the stone's distinctive red appearance. The chromium concentration directly affects color intensity: optimal levels (1-3% by weight) produce vivid red, while excessive chromium can darken or muddy the color.
Key facts
| Chemical formula | Al2O3 with Cr3+ dopant |
|---|---|
| Field | Inorganic Chemistry |
| Color mechanism | Chromium(III) d-orbital absorption of blue-yellow wavelengths |
| Hardness | 9 on Mohs scale (second only to diamond) |
High-quality Burmese rubies contain approximately 1-3% chromium oxide within the corundum matrix. These stones are prized for their pure "pigeon's blood" red hue, which results from optimal chromium doping and minimal iron or titanium impurities that would affect the color.
Frequently asked questions
What causes ruby's red color at the atomic level?
Chromium(III) ions in the corundum lattice absorb blue and yellow light wavelengths (400-600 nm) via electronic transitions between d-orbital energy levels, reflecting or transmitting red light that dominates the visible appearance.
How does ruby differ chemically from sapphire?
Both are aluminum oxide (corundum), but ruby's red comes from chromium dopants at higher concentration, while other sapphire colors come from different impurities: iron and titanium together produce blue, while chromium at concentrations too low to be classified as ruby produces pink. Other trace elements can yield yellow or additional hues.