1. Mineralogical Structure and Crystal Lattice
Corundum is a crystalline form of aluminium oxide (Al₂O₃). In its pure form, corundum is completely allochromatic (colourless, known as white sapphire or leucosapphire). The crystal structure consists of a nearly hexagonal close-packed array of oxygen anions (O²⁻) with two-thirds of the octahedral interstitial sites occupied by aluminium cations (Al³⁺). The space group is R3̄c (No. 167) with lattice parameters a = 4.75 Å and c = 12.99 Å.
2. The Chemistry of Ceylon Royal Blue (IVCT)
The vivid royal and cornflower blue coloration of Ceylon sapphire is not caused by simple crystal field absorption of isolated transition metals, but by intervalence charge transfer (IVCT) between adjacent iron and titanium cations: Fe²⁺ + Ti⁴⁺ + hν → Fe³⁺ + Ti³⁺. This optical transition absorbs yellow-to-red wavelengths (broad absorption band centered at 588–700 nm), allowing predominantly blue light to be transmitted. Ceylon sapphires typically possess low iron concentrations relative to basaltic deposits (such as Australia or Thailand), resulting in exceptional optical transparency and minimal greenish undertones.
3. Thermal Treatment Verification & Inclusions
In unheated Ceylon corundum, rutile (TiO₂) silk needles remain needle-sharp, reflective, and continuous across crystallographic axes. When heated to high temperatures (>1200°C), rutile dissolves into the aluminium oxide lattice or decomposes into dotted discoid droplets. Unheated stones also maintain intact zircon crystals without thermal expansion fractures, and demonstrate sharp hydroxyl (O-H) stretching absorption bands at 3161 cm⁻¹ under Fourier-Transform Infrared (FTIR) spectrophotometry.