Topaz
Physical properties:
| Chemical formula: | Al2 [SiO4] (F, OH)2 |
| Hardness: | 8 |
| SG: | 3.53-3.56 |
| Cleavage: | Perfect, one direction |
| Fracture: | Fairly brittle |
| Thermal sensitivity: | None |
Optical properties:
| Refractive index (RI): | 1.61-1.638 |
| Dispersion: | 0.014 (medium) |
| Double refraction: | 0.008 (weak) |
| Pleochroism: | Variable |
| Critical angle: | 38.5° |
Facet angles (pavilion/crown):
| Vargas: | 42°/41° |
| Olson: | 40°/43° |
| Soukup: | 39°/43° |
| Roth: | 39°/43° |
| MDR: | 39°/43° |
| Schlagel: | 40°/36° |
| Hashnu: | 42°/41° |
| Sinkankas: | 40°/40° |
| GIA: | 39°/43° |
| Perkins: | 39°/43° |
| Raytech: | 40°/39° |
| Cornwall: | 42°/35° |
| Weikoff: | 42°/39° |
| Carroll: | 42.6°/35.3° |
Cutting lap:
All grit sizes
Facet polishing:
| Vargas: | Aluminum oxide on tin or tin/lead |
| Olson: | Aluminum oxide on tin |
| Soukup: | Aluminum or tin oxide on tin or tin/lead |
| Christiansen: | Aluminum oxide on tin or tin/lead (high speed) |
| Perkins: | Diamond on BATT or Corian |
| MDR: | 14-50k diamond on zinc, Aluminum or cerium oxide on acrylic |
| Raytech: | Aluminum oxide on Fast Lap, tin or phenolic |
Cabochon polishing:
| Christiansen: | Aluminum oxide or diamond 50k or 100k on leather or wood |
Orientation:
Very important! The basal cleavage plane should not be parallel to any facet. Orient the cleavage at approximately 10° from the table facet or the perpendicular line. Take care of the culet.
Treatments/synthetics:
Not synthesized. However, synthetic spinel is often sold with the "right" color as topaz. Brown/orange can be heat-treated to white and irradiated to blue. Almost all blue topazes (sky, London, Swiss, electric, neon, etc.) are irradiated and heat-treated.
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