Phenomenal corundum (rubies & sapphires)

Color change sapphire

                    Color-change sapphires are corundum’s chameleons---stones that change colors under different lighting. Under daylight-equivalent (fluorescent) light, the typical color-change sapphire’s basic hue ranges from blue to violet. Under incandescent light, it ranges from violetish purple to strongly reddish purple. Some ‘Rare’ color change sapphire sapphires change in green in daylight to reddish brown in incandescent light. When gem experts judge color-change sapphires, they describe the color change as weak, moderate, or strong. The strength of the stone’s color change is the important quality factor affecting its value, follow by bodycolor. The trace element ‘vanadium’ causes color change in sapphire. Where corundum forms, there’s rarely enough vanadium present to create color change, so these sapphire are very 'Rare'.

Star corundum (rubies & sapphires)

            Star rubies and star sapphire belong to the phenomenal category because of the star-like rays that intersect across the stone’s faces. The star effect is called asterism. It is caused by reflections from tiny, needle-like inclusions that are oriented in several specific directions. Stars are usually made up of 2, 3 or 6 intersecting bands, resulting in 4, 6, or 12 rays. The most common stars have 6 rays, and 12 rayed stars are quite ‘Rare’. When corundum rough contains enough silk to display a star, the cutter or manufacture has to decide whether to fashion the stone as a cabochon and show the star, or heat-treat it to dissolve the and then facet the gem. The decision is an economic one. Faceted corundum typically sells for more per carat then star corundum fashioned as a cabochon. For this reason, most transparent rubies and sapphires on the market are heat-treated to remove any existing silk and to improve bodycolor. However, if the stone’s bodycolor before heat treatment is exceptional, and good-quality asterism is visible, the sapphire might be left untreated and sold as a ‘star’.

 Black star sapphire

          In black star sapphires, hematite inclusions cause asterism. The sapphire’s bodycolor is actually yellow, green or blue, but the inclusions make it appear dark brown or black.

Treatment

          Manufactures can use surface diffusion to create asterism in corundum cabochon. They pack the gems in a mixture of iron and titanium oxides, and then heat them for long periods of time. Slow, controlled cooling. The result is that titanium and oxygen combine into a shallow layer of rutile (needle-like inclusions). The treatment is rarely done, and the star that results is shallow and can be removed with polishing.


Christina Win