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Phenomenal Gems
Copyright © Barbra Voltaire, FGG, G.G. 1999
"Fluorescence" in common parlance refers to the emission of visible light from a substance being irradiated by ultraviolet light, which the human eye cannot see. Some minerals, for example, fluoresce in the infrared, but we cannot see the emitted light. Moreover, the agent that causes a given substance to fluoresce need not be ultraviolet light-it could just as well be an electron beam, or X-rays, or even visible light of a different wavelength from that emitted. All fluorescence, however, involves the addition of energy by some means to a substance, and the reemission of part of that energy as electromagnetic radiation.
Color Change
The color change phenomena is a result of the presence of trace amounts of vanadium and chromium +3 ions. These trace elements apparently effect the absorbtion bands of the mineral. When the light has a greater amount of ultraviolet(daylight), the stone will be blue or green, but when the light source is reddish (incandescent), the stone appears red, purple or raspberry.
Asterism is caused when needle-like inclusions orient themselves perpendicular to a crystal face. In sapphire and ruby, a hexagonal crystal, the star will appear 6 sided. In diopside, an orthorhombic crystal, the star appear 4 sided.
An effect seen on certain minerals which causes it to display a billowy, rounded, ghost-like reflection
with a bluish-whitish color emanating from the surface when the mineral is cut into a cabochon. It is caused by structural anomalies or build up of water in the mineral. The minerals most famous for exhibitingadularescence are Opal and Moonstone. The name is derived from Adularia,
of which Moonstone is a variety of.
Parallel needlelike inclusions of microscopic size
that reflect a streak of light as in Tiger's eye, Pietersite, & Charoite. A chatoyant stone with just a slight change in viewing angle changes it from light to dark

Tenebrescence is a remarkable property of certain minerals to temporarily change their color.
Very few minerals exhibit tenebrescence, also known as reversible photochromism, but at least two do, hackmanite, a variety of sodalite, and tugtupite.
Such behavior is exploited in synthetic materials for the manufacture of self-adjusting sunglasses, which darken on exposure to sunlight.