1. Optical Physics of Chromium (Cr³⁺) Coloration
In ruby, trivalent chromium ions (Cr³⁺) substitute for aluminium ions (Al³⁺) within the octahedral sites of corundum. Crystal field theory explains that Cr³⁺ experiences an octahedral ligand field that splits its 3d electron orbitals, absorbing green-yellow (550 nm) and violet (410 nm) wavelengths, leaving red transmission windows at 600–700 nm.
2. The Iron Quenching Mechanism and Ceylon Glow
Unlike basalt-hosted rubies (from Thailand, Cambodia, or Mozambique) which carry significant iron (Fe³⁺) impurities that extinguish or 'quench' red fluorescence, Ceylon rubies formed in low-iron metamorphic marble and granulite horizons. As a result, incident daylight and long-wave ultraviolet (365 nm) excite Cr³⁺ electron states into intense red emission fluorescence (the R₁ and R₂ emission lines at 694.3 nm and 692.9 nm), imparting a luminous internal glow.
3. Dichroism and Optimal Lapidary Cutting
Ruby displays distinct dichroism: purplish-red parallel to the optic axis (c-axis) and orangey-red perpendicular to the c-axis. Expert lapidaries in Sri Lanka orientation-cut rubies with the table facet perpendicular to the c-axis to concentrate the pure spectral red hue and maximize light return.