Opal
Opal is a water-containing amorphous silica, sometimes with small areas of cristobalite embedded. These microscopic spheres are arranged in a lattice and create reflections in different colors (play of color). An opal with or without play of color, transparency, and in the color range of light yellow-orange-red is called fire opal.
Physical properties:
| Chemistry: | SiO2 · nH2O |
| Hardness: | 5.5-6.5 |
| SG: | 1.98-2.5 |
| Cleavage: | None |
| Fracture: | Very brittle |
| Heat sensitivity: | High |
Optical properties:
| RI: | 1.45 (1.37-1.52) |
| Dispersion: | None |
| Double refraction: | None |
| Pleochroism: | None |
| Critical angle: | 43.5° |
Facet angles (pavilion/crown):
| Vargas: | 44°/43° |
| Olson: | 43°/42° |
| Soukup: | 45°/41° |
| Roth: | 45°/41° |
| MDR: | 45°/41° |
| Schlagel: | 45°/41° |
| Sinkankas: | 43°/40-50° |
| Perkins: | 45°/41° |
| Cornwall: | 45°/39° |
| Weikoff: | 44°/42° |
| Carroll: | 44.5°/31.2° |
Cutting lap:
600/1200. Pre-polishing with 1200 bonded or 3k metal/composite.
Facet polishing:
| Vargas: | Cerium oxide on acrylic, Aluminum oxide on tin/lead |
| Olson: | Tin/cerium oxide on acrylic or tin |
| Christiansen: | Cerium oxide on acrylic, Aluminum oxide on tin/lead |
| Herbst: | Cerium oxide |
| Perkins: | Cerium oxide on Corian |
Cabochon polishing:
| Olson: | Tin oxide on felt |
| Christiansen: | Cerium oxide on felt, tin oxide on leather or wood |
| Cox: | Cerium oxide on felt, tin oxide on velvet or muslin |
| Covington: | Cerium oxide on felt, tin oxide on muslin |
Orientation:
After shape.
Treatments/synthetics:
Opals with play of color are manufactured in plastic. Natural opals are porous, so they can be easily dyed. Opals with "flashy" base colors are usually colored.
Tips!
It is advisable to let fire opals dry out for at least a year to reduce the risk of cracks or loss of transparency at a later stage.
Do not clean opals with red rubbing alcohol, as they can absorb the color and turn pink.
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