Opalescence effects in Opal

by Nasrina Parvin Khan

The milky, turbid aspects of opal described as opalescence. This phenomenon has been seen in common opal or potch opal. These types of opal do not show a play of color.

There are various kinds of special optical phenomenon, which can be exhibit by the gems. Special optic phenomenon is the unique property of a gem that adds a distinct identity character to the gem. We can identify the gem with this property. Sometimes a particular type of gem shows multiple special optical phenomena, like opal.

We know opal for its play of color. Except play of color, opal exhibits another special optical phenomenon named opalescence.

Usually, people confused with two special optical phenomena in opal. But play of color and opalescence are two different phenomenon of opal.

The milky, turbid aspects of opal described as opalescence. This phenomenon has been seen in common opal or potch opal. These types of opal do not show a play of color.

The term ‘opal’ derives from the Greek ‘opallios’, the Latin ‘opalus’ and the Sanskrit ‘upala’ that means precious stone.

The ranges of color in opal may vary from colorless to white, milky blue, gray, red, yellow, green, brown and black.

Opalescence – How it happen?

Opalescence is caused by the reflection of short wavelength light. Usually it occurs in the blue spectrum. Due to this action of light a milky blue, pearly appearance occurs in opal which known as opalescence.

The structural pattern of opal is the reason of this special optical phenomenon thus the reflection of short wavelength.

Opal is an amorphous form of silica (SiO2, nH2O). It is made up of sub-microscopic spheres of silica with a little amount of water. Air also trapped in the spaces between these round spheres. The amount of water in opal may 3% to 21%.

In case of common opal, the openings between the spheres are comparatively large. So, the light doesn’t have to bend when traveling through them and no diffraction can takes place. But the light scattered instead of diffraction. The scattering effect of light creates a sort hazy effect in the inner particles of opal, which is commonly known as opalescence.

This phenomenon is best admired in a flat cabochon-cut opal (a tall, domed shape gem is called as cabochon). The whole surface of opal exhibits opalescence in this case.

Opalescence Types

We can divide opals in various types according to the color and opalescence type, such as i) white opal with opalescence against the pale gray or pale blue background, ii) black opal with effective opalescence and dark background, iii) harlequin opal with spectacular fancy opalescence outline against different colors of mosaic figures, iv) flaming opal of yellow or red color with spectacular opalescence consisted a fire.

Opalescence is the optical phenomenon, which adds value to the opal and helps the gem belong to the royal family.

Other Optical Effects in gemstones
Asterism
Play of Color
Chatoyancy
Labradorescence
Adularescence
Color Change phenomenon
Special Optic Phenomena