This month’s featured stone is a fine example of this color shift.

Sapphire comes to us in many sizes, shapes, colors, and phenomena. Change of color is an optical phenomenon that can be observed as one takes a sapphire from the indoor incandescent light to outdoor sunlight.

Incandescent light is tipped to the longer wavelength end of the visible spectrum, which includes yellow, orange, and red; while sunlight is balanced, emitting the full spectrum. The human eye is sensitive to green light, so we observe more of the blue-green end of the spectrum in daylight, and more of the purple-red under incandescent light.
In sapphire, this unusual selective-absorption behavior takes place when the stone has a

Sapphire

mixture of chromophores. Technically, as Richard W. Hughes has explained in Ruby & Sapphire, these are: (Cr³⁺), which creates the red/purple component, and (Fe²⁺ + Ti⁴⁺), which creates the blue component.
Change of color can occur as a full color change from green to red, or as a color shift from blue to purple. Color change from one end of the spectrum to the other, aka the alexandrite effect, is rarely seen in sapphires and often is caused by the presence of vanadium. Color shift from blue to purple is more commonly seen and can add a beautiful twist to sapphire. This month’s featured stone is a fine example of this color shift.