Quick Links:

Basic Data on Corundum

HARDNESS 9 (1/400 the hardness of a diamond)
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)
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.

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):

Rubies and sapphires are found in Burma, Thailand, Sri Lanka


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).

  • normally use a mixed cut on the crown
  • brilliant and step cut on the pavillion


Undesirable inclusions

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.

Desirable inclusions:

Give rise to asterism (star stones): 'silk'-like texture due to fine rutile needles throughout the crystal.  The light reflects from the fibers -> 6- pointed star.

NOTE: There are 3 special orientations 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

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?

  • Natural origin may be proved by inclusions (e.g., natural gaseous and fluid bubbles) and spectroscopic measurements.
  • Synthetic origin may be indicated by
    • the presence of flux inclusions and non-natural gas inclusions
    • synthetic corundum may contain a visible seed crystal (esp. in older gems)
    • whispy white veils
    • strain cracks, curved striae etc...

    An example of a synthetically created ruby.


    Heat Treatment of Corundum

    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.

    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.

     Diffusion treatment for sapphires is similar to that for any corundum variety. 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.

    some other comments and information

    Gallery of corundum and sapphire images!

    Previous Lecture:  Color in Minerals
    Next Lecture:  Beryl

    Other Tools

    Mineral Reference