Waco Gem and Mineral Club, Waco, Tx

17.7 Other Special Effects


17.7: Other Special Effects

Here, in brief, are a few other special effects not already described that some of your juniors may wish to explore. See if they can discover the reasons behind one or more of these special effects:

Opalescence, fire-and-flash, or play-of-color. These are all terms describing the vivid multicolored dance you see in an opal as you twist and turn it under the light.

Adularescence. When turned in the light, some minerals, like labradorite and moonstone feldspars, exhibit a milky, bluish luster that‘s been compared to the reflection of the moon on water. This is also sometimes called 'moonstone effect'.

Iris or rainbow effect. Some banded agates, when sliced thin and held to the light, exhibit all the colors of the rainbow in a gorgeous, iridescent display.

Phosphorescence. Some fluorescent minerals momentarily holding a faint glow after a fluorescent lamp has been switched off, with the glow gradually fading away in the dark. It‘s especially noticeable in pink calcite rhombs from Nuevo Leon, Mexico. This also sometimes is called 'afterglow'. (See Activity 18.6.)

Thermoluminescence. This is a faint glow, similar to phosphorescence, created when a mineral like fluorite is just mildly heated, well below the point of incandescence.

Pleochroism or change-of-color. 'Pleochroism' means 'any colored'. Some minerals (alexandrite, sapphire, tourmaline, benitoite, etc.) may change colors when viewed from different angles. For instance, some benitoite crystals may be blue viewed from above yet colorless viewed from the side. Dichroic minerals will show two colors; trichroic minerals show three.

Tenebrescence. Some minerals, like hackmanite, change color after exposure to ultraviolet light, then fade under daylight, only to regain brighter colors with a little more UV exposure. This property of reversing color with changes in light radiation is called tenebrescence. (See Activity 18.6.)

Shooting sparks. Striking a chunk of pyrite or of flint against a piece of steel will create sparks.

Aventurescence and schiller. Particularly when polished, aventurine (massive quartz or feldspar filled with tiny plate-like inclusions of mica, rutile, or hematite) glistens as if filled with metallic confetti frozen in mid-air.

Most popular guidebooks about rocks and minerals, aimed at kids and adults, have a section or chapter devoted to special effects. For example, here are a few I know about:

Bonewitz, Smithsonian Rock & Gem, 2005.

Farndon, The Complete Guide to Rocks & Minerals, 2006. See the section 'Mineral Properties: Optical'.

Symes, Eyewitness Rocks & Minerals, 2004.

Ward, Phenomenal Gems, 2008. This great little book is filled with colorful photos showing gems that glow, shimmer, and change color, along with explanations.

Steer kids to books like these and have them explore even more special effects above and beyond those listed here.