Sapphire
Corundum in any color of the mineral except red. Nowadays, sellers often like to call even pink, violet, and orange-red corundum "ruby" instead of sapphire. From a lapidary perspective, the nomenclature is not of great importance.
Physical properties:
| Chemistry: | Al2O3 |
| Hardness: | 9 |
| SG: | 4.00 |
| Cleavage: | None |
| Fracture: | Conchoidal, tough |
| Thermal sensitivity: | None |
Optical properties:
| RI: | 1.76–1.77 |
| Dispersion: | 0.018 (medium) |
| Double refraction: | 0.008 (weak) |
| Pleochroism: | Strong in deep colors |
| Critical angle: | 34.5° |
Facet angles (pavilion/crown):
| Vargas: | 42°/40° |
| Olson: | 39°/37° |
| Soukup: | 42°/37° |
| Roth: | 42°/37° |
| MDR: | 42°/37° |
| Schlagel: | 40°/36° |
| Hashnu: | 42°/40° |
| Sinkankas: | 40°/40° |
| GIA: | 42°/37° |
| Perkins: | 42°/37° |
| Raytech: | 40°/38° |
| Cornwall: | 40°/34° |
| Weikoff: | 42°/38° |
| Carroll: | 40.8°/33.4° |
Cutting disk:
All grit sizes below 1200 (just 1200 often cause problems with "glazing" or "orange peel"). Pre-polish with 3k-14k.
Facet Polishing:
| Vargas: | 100k diamond on ceramic disk or tin |
| Olson | 14k diamond on tin or copper |
| Soukup: | 14k diamond on copper, zinc, or tin |
| Christiansen: | 50-100k diamond on tin or copper |
| Herbst: | Diamond on metal or ceramic |
| Perkins: | Diamond on BATT |
| Raytech: | 14k or finer diamond on Fast Lap, ceramic, tin, zinc, copper, or phenolic |
| MDR: | 14-50k diamond on tin |
Cabochon Polishing:
| Olson: | 8-14k diamond on wood |
| Christiansen: | 50-100k diamond on wood |
| Cox: | 8-14k diamond on wood, plastic, or crystalpad |
Orientation:
For best color and yield.
Treatments/Synthetic:
Some treatments give good color, but only on the surface. Be careful during repair cutting! Synthetic sapphires are on the market that have been treated to look like natural rough.
Tips!
Corundum is very hard and generates high heat during polishing. Have a small cup of water handy and dip the stone every 5 seconds. Corundum can also be polished with aluminum oxide, which some cutters finish with to give the polish an extra boost.
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