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°

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.

Sapphire rough

Sapphire on Gemdat


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