|Cleavage||None, may show parting on twinned stones|
|Optic nature||Uniaxial -|
|Refractive index||1.762-1.770 |
|Pleochroism||Moderate to Strong|
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.
- 1 Diagnostics
- 2 Phenomena
- 3 Synthetics
- 4 Inclusion Images
- 5 Occurence
- 6 Sources
- 7 External links
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.
Translucent to opaque
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.
Optic sign is negative like most, naturally occuring, uniaxial gemstones.
The specific gravity of sapphire is between 3.98 and 4.02 (mean = 4).
It will sink is all common used heavy liquids.
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 460nm (less sharp than the 450nm line) will be accomanied with a third (sometimes faint) line at 470nm. Due to partial absorption of wavelengths between these 450 and 460nm lines, this whole section may blend together with "smudges" between the lines.
With lesser iron content only the 450nm line may be observed in natural blue and yellow sapphire. Although this same 450nm line can also be observed in some blue flame fusion (Verneuil) synthetic sapphire, the "450 complex" has not been reported for synthetic sapphire.
Most natural sapphires are cut with the table almost perpendicular to the optic axis and an 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.
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 (mostly from Thailand) 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.
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 475nm (in the blue). In some rare cases this 475 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
Kashmir, Burma, Sri Lanka, Madagascar, Australia, China, Montana USA, Thailand
- A students' guide to spectroscopy (2003) - Colin H. Winter
- Ruby & Sapphire (1997) - Richard W. Hughes ISBN 0964509768