Optical Special Effects in Natural Gemstones

There area few gems which can display spectacular color and other effects under certain lighting conditions. These effects are often rare and desirable.
Here are definitions for some terms you may encounter while searching for gemstones...

Iridescence/Iridescent: refers to a rainbow-like effect caused by a phase shift caused by semi-transparent layers in a material. Example of material which exhibits this effect: Aurora Borealis finish crystal
Dichroism/Dichroic: refers to a shift between 2 colors (for 3 it would be called Trichroic) depending on the angle from which the stone is viewed and/or the faceting. This is caused by the structure of the crystal, which causes the light to be polarized (filtering out certain colors). Most material marketed as "Dichroic Glass" fits more into the definition of Iridescent, as they tend to display a rainbow effect, rather than true dichroism. Example gemstones which can sometimes exhibit a dichroic effect: Oregon Labradorite Sunstone, Tourmaline and Andalusite
Oregon Sunstone - red, green and gold trichroism

Chatoyant/Chatoyancy: terms which describe a fiberous shimmer as the light moves across the stone - similar to how light reflects off a spool of metallic thread or finewire.The term gees from the French for "cat's eye." Example gemstones which exhibit this effect: Tiger's Eye,Charoite, Seraphinite, and various stones which have a "cat's eye" effect.
Labradorescent/Labradorescence: this is an Irridescent effect which appears as a highlight when a light source moves across a stone. Example gemstone which exhibits this effect: Spectrolite
Labradorite/Spectrolite

Aventurescent/Aventurescence: refers to sparkles which flash as light travels across a stone. Example gemstone which exhibits this effect: Aventurine
Asterism: refers to bright lines or bands which appear when a strong light strikes a stone from a certain direction. This is caused by rutile or other mineral inclusions which diffract the light along the axes of these crystals. In some gemstones, this can produce a star, and in others a band or bands (i.e., a Cat's Eye, which some term Chatoyant - French for "cat's eye"). Examples of gemstones which can exhibit this effect include: Star Sapphires, Star Garnets and Binghamite
Star Ruby/Corrundum

Schiller: refers to layers or plates of metallic inclusions which appear (flash) only when viewed from certain angles. The effect depends on the fineness and orientation of the inclusions. Example of gemstones which can exhibit this effect: Various Sunstone Types
OregonLabradorite Sunstone with Copper Schiller

Adularescent/Adularescence: refers to an effect where a glow moves within a stone as the light travels across its surface. Example gemstone which exhibits this effect: Moonstone
Rainbow Moonstone

Opalescent/Opalescence and Pearlescent/Pearlescence: refer to a soft sheen seeming to gee from beneath the surface of a stone. Example gemstone which exhibits this effect: Pearl
Opalescent Valentinite

Pleochroic/Pleochroism: a general term referring to double refracting crystals which show a Dichroic or Trichroic effect.
Color Change: denotes a stone which changes color markedly depending on the wavelength of the light striking it. E.g., a stone which is green under daylight, might be red when viewed under incadescent light. This is caused by types of polarization which absorb certain wavelengths of light. Example gemstones which exhibit this effect: Sapphire, Alexandrite and Garnet
Color Change Garnet (incadescent light on left, sunlight on right)

Color Play or Fire: refers to intense color thrown off from within a stone. This is similar to Labradorescence, only the color occurs in localized bands, bubbles, platelets or areas. Examples of gems which display this effect include: Black Opal and Fire Agate
Fire Agate

Fluorescent/Fluorescence: a color change and/or glow when viewed under ultra-violet light or x-rays. Example gemstones which sometimes exhibit this effect: Sodalite, Fluorite and Scapolite
Tenebrescent/Tenebrescence: refers to a stone which changes color upon prolonged exposure to light, and which reverts to the original color after being removed from light (the same effect as is used in those self-darkening prescription lenses). Example gemstones which can sometimes exhibit this effect: Hackmanite and Tugtupite
Tugtupite (fully UV-charged in photo on right)

Phosphorescent/Phosphorescence: refers to a glow displayed by some materials after light is removed. Most phosphorescence in gems is too weak to be directly observed. Some man-made jewelry have this property, however.
Notes: - Generally, the stronger the effect, the more desirable the gem will be to collectors.
- Quality of the cutting can make the difference between an intense, interesting display, and no display at all. A bad cut can result in a weak, confusing effect which muddies the stone.
- Some stones are purposely cut to hide the optical effect, often when the effect degrades the color or clarity of the gem.
- As well as natural gems, many man-made materials have been produced which imitate these effects (some more successfully than others). Most of the man-made effects are done in glass work, though some are also applied as coatings to less desireable gemstones.
- Be aware that some optical effects can be ruined by improper handling. Never expose your gemstones to heat or chemicals, both of which can impair or destroy the display. Clean using warm (not hot), soapy water, followed by a quick rinse in warm water. Avoid ultrasonic and similar devices, which can fracture gems which contain inclusions that result in the optical fireworks.

Other subjects on which I've written recent Guides...Other GemstonesCoin Collecting
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