Basic Data on Corundum
*Note: some texts list the crystal system of ruby as trigonal. Trigonal is more simply considered a subdivision of the hexagonal crystal system.
Buying rubies: what to consider?
Sapphires:Physical characteristics: composition, structure etc. same as for a ruby.
Color (other than ruby red)
The blue color is due to charge transfer involving Fe-Ti (see lecture on color in minerals for details!.
Different concentrations of various impurities produce a range
of colors from quite
pale, due to low concentrations, to quite
deep blue. Other colors include:
yellow, orange, green, etc.
|What is corundum?
Where is corundum formed and found?
How are crystals cut?
Star rubies and sapphires
Natural versus synthetic?
Origin of Rubies and Sapphires (Corundum):
Cutting:Cutting refers to the proportions and finish of a gem, regardless of the shape or size. In other words, did the cutter do a good job? Are the facets (polished faces) placed symmetrically? Are they smooth, or do they have minute pits and lines? Are the facet junctions crisp, and do the facets meet correctly? Is the pavillion (the bottom) of the stone of sufficient depth that you see bright reflections across the entire face of the stone? A stone that is shallow "leaks" light out the bottom and is not brilliant. This is called "windowing"--you can see right through the stone, like through a window.
More details on the importance of refractive index and critical angle are provided in the first lecture
Crystals normally cut with table of cabochon perpendicular to long axis of crystal (long axis is axis about which crystal has hexagonal symmetry) results in best color (because it is pleochroic).
star can change positions depending on where the light hits
Natural versus synthetic
Rubies and sapphire are commonly synthesized by the Verneuil method
It is possible to synthesize both clear crystals and stars. The first synthetic rubies appeared on the market in 1908.
Usually, a single crystal or boule is grown from a melt by one of several methods.
How do I tell if the stone is synthetic?
An example of a synthetically created ruby.
Heat Treatment of Corundum
Heat treatment of gemstones to is done to improve their appearance.How do I tell if a stone has been heat treated?
Detection of heat and diffusion treatment is possible because these treatments modify natural inclusions. This may involve rupture of gas or fluid inclusions or partial dissolution of mineral inclusions. For gems that contained needles, the needle margins may become diffuse.
Diffusion Treatment of Corundum
Why? Color enhancement can be achieved through addition of the color-causing impurity to the surface of the faceted gemstone. For Ruby, this involves heating the stone to very close to its melting point in the presence of a chromium source (chromium oxide powder: Cr2O3). Chromium enters into the structure of the corundum (diffuses into the corundum). This is a slow process, so chromium enrichment only occurs in the surface layer. This is sufficient to produce a strong color enhancement that is difficult to detect by eye.
There are several ways you can determine if your stone has been treated. Diffusion treatment will result in concentration of color at facet junctions, and will modify the refractive index of the gemstone.
|some other comments and information|
of corundum and sapphire images!
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