CORUNDUM
TOPICS:
BASIC DATA ON CORUNDUM
| COMPOSITION | Al2O3 |
| HARDNESS | 9 (1/400 the hardness of a diamond) |
| CRYSTAL SYSTEM | Hexagonal* |
| INCLUSIONS | Common (minerals and glass) |
| SPECIFIC GRAVITY | High (around 4.0) |
*Note: some texts list the crystal system of ruby as trigonal. Trigonal is more simply considered a subdivision of the hexagonal crystal system.
| COLOR | Red (ruby is Latin for red), may also be pinkish or brownish-red; |
| (absorbs blue, transmits and fluoresces red) | |
| PLEOCHROISM | Strong |
| IMPURITIES | Red: Cr+++ (< 1%), Brown: Fe+++ |
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:
purple and pink, yellow,
orange, green, etc.
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).
While many cut rubies and sapphires contain inclusions, gems that have eye-visible inclusions are less desirable than "eye-clean" stones. In some cases, inclusions can make the stone more vulnerable to breakage.
NOTE: There are 3 special orienatations in which rutile crystals occur. These are parallel to hexagonal faces and thus
at
60
degrees.
Note that the chemical formula of rutile is TiO2, and Ti is one of the
elements
responsible for color in some sapphires
Another example of a star Ruby
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?
Heat treatment of gemstones to is done to improve their appearance.
Heat treatment may change the color of corundum for a variety of reasons.
In some cases, heating the stone causes changes in the oxidation state
of impurites. An especially important example involves reduction of Fe
(conversion of Fe+++ to Fe++). Fe++ causes color in a variety of sapphires
via charge transfer (see lecture on color
in minerals).
Conversely, stones that are too deeply blue may be lightened by oxidation of Fe (converstion of Fe++ to Fe+++)
In some cases, heat treatment will improve the depth of color because
heat causes dissolution of inclusions and diffusion of impurites
(especially Ti from rutile inclusions) into the surrounding corundum. Because
fine inclusions cause some stones to look cloudy, heat treatment
that dissolves the inclusions may also improve the clarity of the stone.
Heat treatment may also remove local color concentrations (remove patches
of color) because heat allows the color-causing impurity (Cr, in the case
of ruby) to more evenly distribute through the crystal. (however, this may require such long times that it is impractical)
The conditions for heat treatment vary, depending
upon the individual stone. Some typical values and conditions are
listed here
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.
Themelis states that "glass-appearing" inclusions may be found on rubies
that have been heat-treated with borax-based substances
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.
Diffusion treatment for sapphire is similar to that for corundum. However,
to enhance the blue color Fe and Ti oxide powders are placed in contact with
the faceted gem and Fe and Ti diffuse into the surface of the stone.
Note that diffusion treatement is done to faceted stones and is probably
not obvious by inspection under normal
viewing conditions!!
Some specifics for conditions for sapphire and ruby treatments are given in this table.
How do I tell if a stone has been diffusion treated??
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.
An excellent, simple, non-destructive method is as follows:
- place the faceted gem
in methylene iodide. Note that color concentrations
are apparent at facet junctions (where the gemstone is thin). This color
concentration tells us that the deeply colored layer is quite thin!
This is how these gemstones look under normal viewing conditions!
To demonstrate that the layer is thin, we polished off
a part of
the girdle region, exposing the pale, untreated interior.
(B) SAPPHIRES:
(2) Origin of Rubies and Sapphires (Corundum):
(3) Cutting:
(4) Clarity:
(A) UNDESIRABLE INCLUSIONS
ASTERISM 'silk'-like texture due to
fine rutile needles throughout the crystal
(B) DESIRABLE INCLUSIONS: Give rise to asterism (star stones)
(5) Natural versus synthetic
Rubies and sapphire are commonly synthesized by the Verneuil method
(6) Treatments:
Heat Treatment of Corundum
Diffusion Treatment of Corundum
Gallery
of corundum and sapphire images!
Questions on the lecture
"Beryl"
To gemstones section
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