Color Change (Sapphire)
Color: Purple, violet to red-violet, blue/purple, pink/red to blue/purple… about any combination is possible
Moh’s Hardness: 8.5-9
Refractive Index: 1.766-1.774
Critical Angle: 34.62°
Specific Gravity: 3.99-4.01
Heat Sensitivity: Can be… but is usually not a problem
Dichroism: strong, especially in darker colors
Birefringence (double refraction): Weak (.018)
Crystal structure: Hexagonal, trigonal, dipyramidal
Treatments: Assume about any Sapphire you see has been heated unless otherwise noted from the supplier. Heat treating to reduce silk is and improve colors is standard. Glass-filling voids in rubies (any sapphire) is not considered an acceptable treatment, by me at least.
What I prefer to polishing with: Zinc laps with 8,000 pre-polish and 50,000 polish.
Jeff’s Comments: Orientation for the best color can take experience and is often not easy for a beginner. Be sure and examine your rough in all lighting conditions when deciding orientation for cutting. Be careful… Sapphire cuts and polishes faster on some sides of the crystal because the hardness varies depending on the direction. It typically will have some pits on some facets (depending on the crystal direction) that need to be polished out. Orientating on the “c” is usually better color. Quality rough is difficult to come by…
Synthetic is usually called corundum and is available in many colors.