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Sapphire
Sapphire is a gem-variety of the mineral corundum, colored in blue with light purple or green hues or without them in Russia. The Russian name comes from the ancient Greek "Σάπφειρος" - a stone of blue color. Blue sapphires are graded basing upon the purity of their primary hue – blue sapphire in world practice.
Chemistry: Al2O3;
Crystal system: Trigonal, ditrigonal-skalenohedral crystal class (point symmetry group);
Color: Blue, blue with a purple hue, blue with a faint green hue.
The blue color is due to trace amounts of elements such as Fe2+ – Ti4+ in the lattice of the mineral.
Identification properties
| Physical properties | |
|---|---|
| Mohs hardness: | 9 |
| Density: | 4.00±0.03 g/cm3 |
| Cleavage: | none |
| Fracture: | conchoidal |
| Optical properties | |
|---|---|
| Optical character: | anisotropic, uniaxial, negative |
| Refractive Index: |
ne = 1.760-1.764, no = 1.768-1.772 |
| Birefringence: | 0.008 – 0.009 |
| Pleochroism: | from weak to distinct in violet, yellow, green tones |
| Dispertion: | 0.018 (BG) |
| Luster: | vitreous |
Inclusions and structural inhomogeneities
Gem basic treatments
| Treatment | Goal |
|---|---|
| Heat treatment | Removing of "undesirable" hues (greenish, yellowish, purple), as well as for the conversion of boehmite microinclusions that enhance the clarity. |
| Heat treatment in a flux | Healing of fractures (cracks). |
| Hydrothermal build-up of a layer of synthetic sapphire | Improving the apparent clarity and color of gem material |
| Diffusion treatment (both surface and diffusion in full scope) | Improving of blue color |
| Diffusion treatment | Lightening of excessively dark color. |
| Healing of fractures (cracks) or impregnation with high refractive glasses or colored compositions to fill the open cavities | Improving the quality. |
| Surface coating by applying a various coloring agent | Altering the gem visual color |
Inclusions and structural inhomogeneities
Heat treatment
Filling cracks
Diffusion treatment
Synthetic or Imitation gem materials
Synthetic sapphire for the jewelry industry is produced by several methods based on different physical principles:
- flame fusion method (Verneuil method);
- "pulling" process from the melt (the Czochralski method, etc.);
- zone melting methods (methods of Bagdasarov, Stockbarger, etc.);
- crystallization methods from a solution in a melt (a group of flux methods);
- crystallization methods from aqueous solution (hydrothermal method group).
The most commonly encountered synthetic sapphire is as follows:
- corundum colored by filling fractures;
- coated corundum;
- corundum with a layer of artificial blue sapphire;
- diffusion-treated corundum (both coating and dyeing);
- doublet: corundum + glass and corundum + garnet+ paste (glass);
- natural and synthetic minerals with a similar color;
- color glass.
Inclusions and structural inhomogeneities
