16.3.2: Sapphire
- Page ID
- 4026
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|---|---|
| Chemical composition | Al2O3 |
| Crystal system | Trigonal |
| Habit | Prismatic, tabular |
| Cleavage | None, may show parting on twinned stones |
| Fracture | Conchoidal |
| Hardness | 9 |
| Optic nature | Uniaxial - |
| Refractive index | 1.762-1.770 (+.009,-.005) |
| Birefringence | 0.008-0.010 |
| Dispersion | Low, 0.018 |
| Specific gravity | 4 |
| Lustre | Vitreous |
| Pleochroism | Moderate to Strong |
Figure \(\PageIndex{1}\): Cornflower Ceylon sapphire
| Sapphire image gallery |
Sapphire (a corundum variety) is an aluminum oxide occurring in every color of the rainbow. It is a stone of great hardness and durability. It can also have phenomenal characteristics like asterism (star sapphire) and color changing (like alexandrite). The color changing varieties are mesmerizing, having the ability to change color depending on whether they are viewed in daylight or incandescent light.
The name is derived from the Greek word "sappheiros" meaning "blue". The history of sapphire dates back to at least the 7th century BC, when they were used by the Etruscans. The sapphires used by the Etruscans, Greeks, and Romans were imported from India (what is now Sri Lanka). Sapphires were reputed to protect kings from harm and envy. In the 13th century, it was written that sapphires had the power to protect against poverty, to make a stupid man wise and an irritable man good-tempered.
Diagnostics
Color
Sapphire occurs in many colors ranging from colorless to black. When a corundum variety is termed "sapphire" it indicates the blue variety. Any other color (except red, which is named ruby) will have a prefix before "sapphire", such as "yellow sapphire".
Causes of color:
- sapphire (blue) - charge transfer between Ti and Fe.
Diaphaneity
Transparent to opaque
Refractometer
Sapphire has a refractive index range between nω = 1.767-1.772 and nε = 1.759-1.763, with a maximum birefringence of 0.009.
The optic sign is negative like most, naturally occurring, uniaxial gemstones.
Specific gravity
The specific gravity of sapphire is between 3.98 and 4.02 (mean = 4).
It will sink in all common used heavy liquids.
Spectrum
Figure \(\PageIndex{2}\): Spectrum of blue, green and Australian yellow sapphire (high iron content)
Natural blue, green and yellow sapphires with a high iron content may show the typical "450 complex" as seen in this image. The clear lines at 450 and 460 nm (less sharp than the 450 nm line) will be accompanied with a third (sometimes faint) line at 470 nm. Due to partial absorption of wavelengths between these 450 and 460 nm lines, this whole section may blend together with "smudges" between the lines.
With lesser iron content only the 450 nm line may be observed in natural blue and yellow sapphire. Although this same 450 nm line can also be observed in some blue flame fusion (Verneuil) synthetic sapphire, the "450 complex" has not been reported for synthetic sapphire.
Polariscope
Most natural sapphires are cut with the table almost perpendicular to the optic axis and a uniaxial interference figure should be easily found. Some synthetics (Verneuil type) are cut with the table parallel to the optic axis and the interference figure will be hard to find on the girdle.
Finding an interference figure on the table is not diagnostic.
Phenomena
Asterism
Sapphire may show 6 pointed or 12 pointed stars.
The 6 pointed rays are from reflections on rutile needles that form in directions parallel to the 2nd order prism. 12 pointed rays form from reflections on rutile needles (2nd order prism) and from reflections of hematite-ilmenite needles that lie in the planes parallel to the 1st order prism.
Color change
Cat's-eyes
Trapiche
Synthetics
Flame fusion (Verneuil)
Color change flame fusion sapphire will have a characteristic absorption spectrum with a fuzzy band in the yellow and a diagnostic line at 473 nm (in the blue, called the vanadium line) In some rare cases this 473 line may be seen in natural sapphire.
Curved growth lines are usually seen.
Other synthesizing methods
- Flux melt (Chatham, Ramaura)
- Czochralski pulling process
- Float zone method
Occurrence
Kashmir, Burma, Sri Lanka, Madagascar, Australia, China, Montana USA, Thailand
Enhancements
Common enhancements to sapphire varieties:
- Fracture filling - oil, wax or plastic (with dye or without)
- Lead glass filling - removes crack increases weight
- Diffusion treated - heating causing layer on cut stones - enhance color or produce asterism
- Heat treatment - improves or even changes the color, reduces silk and other impurities
- Irradiation - change colorless stones to yellow - color unstable
- Beryllium treatment - Heated with beryllium as an Additive to produce Orange/yellow/blue colors
A short Film on the Sapphire Mining Techniques in Bang Kacha
Bang Kacha, in Thailand, produces predominately green and golden-orange colored sapphires.
Small quantities of blue sapphires, as well as some star stones, are found here.
A short Film on Sapphire Mining in Ilikaka, Madagascar
A short Film on Sapphire Processing in Sri lanka ( ceylon )
This film Shows a trip to the mines of Ratnapura, Pale sapphires of blue and yellow are sorted and heat treated at 1000 C for a few hours to produce White Sapphires. The heat dissolves all silk and Makes the stone whiter and brighter to use as a natural substitute for diamonds.
Sources
- A students' guide to spectroscopy (2003) - Colin H. Winter
- Ruby & Sapphire (1997) - Richard W. Hughes ISBN 0964509768